Unable find real root for Cubic Equation in C++

I am writing a C++ program for finding the real root, x for a cubic equation 〖ax〗^3+〖bx〗^2+ cx+d=0 where a≠0 and b=0.

Unfortunately, I could not output the "test case 1 & 4" (ps. sample output provided below link). Perhaps any logic syntax in my coding? Greatly appreciated if anyone could show me the correct way to do it.

#include <iostream>
#include <cmath>
#include <iomanip>

using namespace std;

int main (){
    int a , b, c , d;
    double x , y;
    double interval1, interval2;
    bool stop;

    b = 0;
    x = 0;
    stop = true;

    cin >> a; 
    cin >> c;
    cin >> d;  

    interval1 = a * pow(x , 3) + b * pow(x , 2) + c * x + d;

    if (interval1 < 0){
        interval2 = interval1 *-1;
    }else{
        interval2 = interval1;
        interval1 = interval2 * -1;
    }

    while (stop=true){
        x = interval1;
        y = a * pow(x , 3) + b * pow(x , 2) + c * x + d;

        if(y>0 && y<0.001){
            break;
        }else{   
            if (x<interval2) { 
                interval1 = x + 0.000001;
            }else{
            stop = false;
            }
        }
    }

    if (x==-0){
        x = 0;
    }

    if(a==0){
        cout << "NOT VALID" << endl;
    }else{
        std::cout << std::fixed << std::setprecision(3) << x;
    }
    return 0;
}

The sample output Pseudocode for the program

Your code is almost done. You need to improve some points.

1. Change int a , b, c , d; to double a , b, c , d;

2. Change condition while (stop=true) to while (stop==true)

I tested and it works as your example.