steve I think we can assume it is B as the other one would probably not be A-level, or at least I cannot see a way of doing it... Or well, I guess it is better OP makes clear, but at least I can't possibly spot a substitution for A...
Yes, both solutions seems correct=) And I think my attempt with by parts will land there as well when I look at it again tomorrow...
What is strange though is that I ran this in mathematica and it suggested another answer...(haven't checked if they are equivalent): 121(cos(3x)−9cos(x))
I did differentiate the answer you've got to and it is correct, so we should be safe=)