I wasn't sure how to solve for $x$ and create the inverse function given the $\cos(2x)$ term. Whenever I tried to take $\arccos$ there was an $x$ on the other side of the equality which meant I was achieving nothing. How do I proceed with this question?
Edit: I'm still nto entirely following. When I apply the formula I get stuck with the $m(\frac{3\pi}{4})$ term. I have a note which says that by inspection, $m(\frac{3\pi}{4}) = \frac{\pi}{4}$ which I don't follow.