1.6（a-b)^2-8（a-b)^3
=2(a-b)^2[3-4(a-b)]
=2(a-b)^2(3-4a-4b)
2.[3a^(n+1)-6a^(n)+9a^(n-1)]/a^(n-1)
=3a^2-6a+9