(x+y)(x+y)^2-(x+y)(2x+y)^2
=(x+y)【(x+y)^2-(2x+y)^2】
=(x+y)【(x+y+2x+y)(x+y-2x-y)】
=(x+y)[-x(3x+2y)]
=-x(x+y)(3x+2y)