functional equations - Math question functions help me?

I have to find find $f(x,y)$ that satisfies
$$\begin{align} f(x+y,x-y) &= xy + y^2 \\ f(x+y, \frac{y}x ) &= x^2 - y^2 \end{align}$$

So I first though about replacing $x+y=X$ and $x-y=Y$ in the first one but then what?