y(x-y)^2+(y-x)^3
=y(x-y)^2-(x-y)^3
=(x-y)^2(y-x+y)
=(x^2-2xy+y^2)(2y-x)
=2x^2y-4xy^2+2y^3-x^3+2x^2y-xy^2
=-x^3+4x^2y-5xy^2+2y^3