-6a^2b-3a^3-3ab^2
=-3a(2ab+a^2+b^2)
=-3a(a+b)^2
(x^2+y^2)^2-4x^2y^2
=(x^2+y^2-2xy)(x^2+y^2+2xy)
=(x-y)^2(x+y)^2