a(a^2+2ab+b^2)+6b^3-6a^2*b
=a(a+b)^2+6b(b^2-a^2)
=a(a+b)^2+6b(b+a)(b-a)
=(a+b)(a(a+b)+6b(b-a))
=(a+b)(a^2+ab+6b^2-6ab)
=(a+b)(a-2b)(a-3b)