求大神帮我解决这个问题，为什么16行里面会会找不到符号

public class Sanjiao{
public double f1(double x,double y){
return y-x;
}
public double f2(double x,double y){
return y + 2*x - 9;
}
public double f3(double x,double y){
return y=1;
}
public static void main(String[] args){
Point p = new Point(2,3);
Point p1 = new Point(4,1);
Point p2 = new Point(1,1);
Point p3 = new Point(3,3);
if(p.f1() * p1.f1()>0 && p.f2() * p2.f2()>0 && p.f3() * p3.f3()>0){
System.out.println("在三角形内部这个传说中的点");
}else{
System.out.println("孩子，不在三角形上哦！");
}
}
}
class Point{
private double a;
private double b;
public Point(double a,double b){
this.a = a;
this.b = b;
}
}