i don't see you have been declare variable x, y
so that the compiler throws that error
to solve that error, you can declare x, y variable inside your function like:
int x = 100;
int y = 100;
int x0 = 5;
int x1 = 2;
....
or write an function like:
public int MyEquation(int x)
{
int y;
return y;
}