Hello this question looked straight forward enough, but I'm stuck and would really appreciate some help.

Solve on the interval

I tried writing it in terms of and

And got this;

, among various similar equations.

I have worked on from here a bit, but can't seem to get it in terms of a trigonometric equation I can then solve. I'd really appreciate it if somone would tell me if I'm on the right track and should keep going or if I need to approach the question in a different way.

Thank you very much.