\(\displaystyle 3x^2y-y^3=b\)

make a formula using a and b for x and y.

What I've done:

\(\displaystyle x^3-3xy^2+3x^2y-y^3=a+b\)

\(\displaystyle x^3+3x^2y-3xy^2-y^3=a+b\)

\(\displaystyle (x-y)(x^2+4xy+y^2)=a+b\)

I don't know what to do from here. Any hints?