As you work through this set of problems, look for patterns.
This one is tricky. Concentrate on the 3x² part of the integrand. What will happen when you anti-differentiate that part?
Refer to part (a) (the question, not the hint).
Before you differentiate, rewrite the expression as with a fractional exponent.
A clue is directly above (in part (c)).
And don't forget the +C.