using integration by parts again?
You couldn't use the power rule on , because wasn't constant. However, because the function in question is a composition of functions, you use the chain rule. You can always use the chain rule on compositions of functions, unless there are domain/behavior issues (not likely encountered in Calc classes; such pathological behaviors are usually reserved for upper level courses in analysis). I like your and your integrand. How do you propose to integrate?
There you go. Now, relating all this back to the problem at hand, the area you are after is equal to the area under the curve minus the area under the triangle. Once you exhibit that difference, you are done with part b, assuming your answer corresponds to the answer required. Does all this make sense?