Thread: Want to decipher a bit of code (pretty new to java)

Hi guys! I needed help deciphering a section of my code. I found it online and got a little help from my friends, but I don't fully understand what it means. My code basically takes in an input from the user in the form mx+b, and then asks for x. The program then solves for x. I need help deciphering section 3 of my code.

 
import java.util.Scanner;
 
class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    //1
    String xExpression;
    System.out.println("Write an expression in mx+b form (eg. 2x+10)");
    xExpression = sc.nextLine();
 
//2 
    int m = Integer.valueOf(xExpression.substring(0,xExpression.indexOf('x')));
    //System.out.println(m);
 
//3
    int index=0, num=0;
    char sign='+';
 
    for (int i=xExpression.indexOf('x');i<xExpression.length();i++) {
      if (xExpression.charAt(i)=='+' || xExpression.charAt(i) == '-') {
        sign = xExpression.charAt(i);
        index=i;
        num = Integer.valueOf(xExpression.substring(index));
        //System.out.println(num);
      }
    }
    System.out.println(num);
 
//4
    System.out.println("Enter x");
    int x = sc.nextInt();
 
    if (sign=='+') {
      System.out.println(m*x+num);
    } else if (sign=='-') {
      System.out.println(m*x-num);
    }
  }
}