(x^2-y^2)(x+y)-(x-y)^3
=(x-y)(x+y)(x+y)-(x-y)^3
=(x-y)(x+y)^2-(x-y)(x-y)^2
=(x-y)[(x+y)^2-(x-y)^2]
=(x-y)(x+y-x+y)(x+y+x-y)
=(x+y)2x2y
=4xy(x+y)