"Let f and g be twice differentiable functions ..." - Derivitive problem

There's a problem that our professor wanted us to look at and I'd really like to understand it. It says

"Let *f *and *g* be twice differentiable functions. Derive a formula for (*fg*'') = [(*fg*)']' using your knowledge of differentiation rules."

We know about the power rule and chain rule and all that fun stuff .. I guess I'm not sure what she's asking.

Am I trying to apply these rules to "[(*fg*)']'" to make it look like (*fg*'')? If I am, I don't know how I would even start that.

Any help would be appreciated.

Thanks