(1)-a^2c^2-c^4+2ac^3
=-c^2(a^2+c^2-2ac)
=-c^2(a-c)^2
(2)(a+b)^4-18(a+b)^2+81
=[(a+b)^2-9]^2
=[(a+b-3)(a+b+3)]^2
=(a+b-3)^2(a+b+3)^2