how can import android.support.v7.app.Appcompatactivity in android studio?

I'm new in coding. I used this import android.support.v7.app.Appcompatactivity for mainactivity.java but I received a red error:

cannot resolve symbol v7

I did invalidate caches restart too but that didn't work. If I run this code without that line, my app runs on Android 5.1.1 but doesn't work on galaxy j6 What's that line for?

My android studio version is 3.5.0.21, I can show my error in a screenshot for more detail too:

package com.example.bmi;

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;
import android.support.v7.app.Appcompatactivity

public class MainActivity extends AppCompatActivity {

    private EditText height;
    private EditText weight;
    private TextView result;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        height = (EditText) findViewById(R.id.height);
        weight = (EditText) findViewById(R.id.weight);
        result = (TextView) findViewById(R.id.result);
    }

    public void calculateBMI(View v) {
        String heightStr = height.getText().toString();
        String weightStr = weight.getText().toString();

        if (heightStr != null && !"".equals(heightStr)
                && weightStr != null  &&  !"".equals(weightStr)) {
            float heightValue = Float.parseFloat(heightStr) / 100;
            float weightValue = Float.parseFloat(weightStr);

            float bmi = weightValue / (heightValue * heightValue);

            displayBMI(bmi);
        }
    }

    private void displayBMI(float bmi) {
        String bmiLabel = "";

        if (Float.compare(bmi, 15f) <= 0) {
            bmiLabel = getString(R.string.very_severely_underweight);
        } else if (Float.compare(bmi,  15f) > 0  &&  Float.compare(bmi, 16f) <= 0) {
            bmiLabel = getString(R.string.severely_underweight);
        } else if (Float.compare(bmi, 16f) > 0  &&  Float.compare(bmi, 18.5f) <= 0) {
            bmiLabel = getString(R.string.underweight);
        } else if (Float.compare(bmi, 18.5f) > 0  &&  Float.compare(bmi, 25f) <= 0) {
            bmiLabel = getString(R.string.normal);
        } else if (Float.compare(bmi, 25f) > 0  &&  Float.compare(bmi, 30f) <= 0) {
            bmiLabel = getString(R.string.overweight);
        } else if (Float.compare(bmi, 30f) > 0  &&  Float.compare(bmi, 35f) <= 0) {
            bmiLabel = getString(R.string.obese_class_i);
        } else if (Float.compare(bmi, 35f) > 0  &&  Float.compare(bmi, 40f) <= 0) {
            bmiLabel = getString(R.string.obese_class_ii);
        } else {
            bmiLabel = getString(R.string.obese_class_iii);
        }

        bmiLabel = bmi + "\n\n" + bmiLabel;
        result.setText(bmiLabel);
    }
}

and my build.gradle code is

apply plugin: 'com.android.application'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.2"
    defaultConfig {
        applicationId "com.example.bmi"
        minSdkVersion 15
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test:runner:1.2.0'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
    implementation 'androidx.recyclerview:recyclerview:1.0.0'
    //noinspection GradlePath
    implementation files('SdkManager\\extras\\android\\support\\v7\\appcompat\\libs and select android-support-v7-appcompat')
}

These are support libraries to made some new features backward compatible. Google moved the support libraries from the android.support packages to androidx. You can't use both, which is why you are getting the error message.

It's encouraged to migrate to androidx. So, you should be fine removing v7 package.

What OS is your j6? What error message are you getting?