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