一开始我也很不理解这X1、Y1、X2、Y2的关系到底是怎么推出来的，y总写的代码也很简洁，隐藏了很多细节，这里说一下我的理解。

大家跟着上面的图推导一下，肯定能理解他们之间的关系。拜拜啦、、、

import java.util.*;
public class Main{
    static long x2;
    static long y2;
    public static void main(String[] args){
        Scanner input = new Scanner(System.in);
        int T = input.nextInt();
        while(T-- > 0){
            int n = input.nextInt();
            int d = input.nextInt();
            int x = input.nextInt();
            int y = input.nextInt();
            int gcd = extGcd(n,d);
            //x2 * n + y2 * d = gcd
            if((y - x) % gcd != 0){
                System.out.println("Impossible");
            }else{
                y2 *= (y-x) / gcd;
                n /= gcd;
                System.out.println((y2 % n + n ) %n);
            }
        }
    }

    public static int extGcd(int a,int b){
        if(b == 0){
            x2 = 1;y2 = 0;
            return a;
        }else{
            int gcd = extGcd(b,a%b);
            long x1 = y2;
            long y1 = x2 - y2 * (a / b);
            x2 = x1;
            y2 = y1;
            return gcd;
        }
    }
}