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?
It's always best to break up a problem into parts whenever you can.
What I would have done was to temporarily cover up the 3a, then expand the parentheses by squaring it, then uncover the 3a to multiply the result from squaring the parentheses and you would have gotten your answer. This method can be adapted towards different algebra problems.