Where is the solution stub in this program and how would I call a method in it?

This is a specific question, don't downvote it just because it doesn't help you.

  public class Answer {
  public static String answer(int n) {
    String nums="";
    int limit = 10005;
    int x=2;
    while(limit>0){
        if(isPrime(x)){
            limit-=String.valueOf(x).length();
            nums = nums + String.valueOf(x);
        }
        x+=1;
    }
    String out="";
    if(n==0){
        out="23571";
    }else{
        for(int i=1;i<6;i++){
            out += String.valueOf(nums.charAt(n+i));
        }
        //Problem Solved: instead of this loop, it should be out = nums.substring(n,n+5)
    }
    return out;
}
public static boolean isPrime(int number) {
    for(int check = 2; check < number; ++check) {
        if(number % check == 0) {
            return false;
        }
    }
    return true;
  }
}

Nothing is wrong with this code as far as I know, I'm just using it as an example for you to use.

"It must implement the answer() method in the solution stub." was in the directions for me, but I don't know much about the vocabulary of programming, I only understand logic behind programming, so this is the only thing I don't know how to solve. So what I am asking is where do I put the "answer()" at in this program?

It was looking for substring, which I didn't include because I haven't used java in about a year and simply forgot about it.

Here as I can figure out you have problems in understanding the meaning of "stub". It is simply the test method as provided by the answer here. And if you want to test the above code you have to implement the main method in your code to do the same. Something like this

 public static void main(String [] args){
  //Either use Scanner object or provide the hard coded input as per your requirements
  Scanner sc = new Scanner(System.in);
  int n = sc.nextInt();
  System.out.println(answer(n));
 }

EDIT AS PER OP REQUIREMENT

Okay so as per your requirement it is asking you for the unit test. There are many ways to do it but my preferred is to make stub concrete class

Implementation an stub concrete class in JUNIT

class Answer {
 public String answer(int n){
    // Code body
  return "result"// in your case out variable
  }
}

class solution extends Answer {
@Override
public String answer(int n){
    //return "your stubbed result";


  }
}