Why am I getting stuck on these questions?

Find and classify the stationary points:

$\displaystyle f(x,y)=(x-y)^3+x^2y^2$

$\displaystyle f_x=3(x-y)^2+2xy^2$

$\displaystyle f_y=-3(x-y)^2+2x^2y$

I forget how to find the roots. What to I need to revise? Any links?