3a(a -2b)^2. It seems to me that you'd just multiply the numbers inside the parentheses by themselves.
That would give you 3a(a^2+4b); the answer to that is 3a^3 +12ab^2. But that's not the right answer.
The right answer is 3a^3 -12a^2b +12ab^2.
So what am I missing?