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?