Given $y=f(\sin x)$, find $\int f'(\sin 3x)\cos 3x dx$.
This is a multiple choice question with four options:
- A) $\quad y+C$,
- B) $\quad \cfrac{y}{3}+C$,
- C) $\quad3y+C$,
- D) $\quad9y+C$.
The figure is my working so far, and the given answer is $B$. I got $\frac{1}{3}f(\sin 3x)+C$ which is totally not in the choices. How should I continue from there? 