Hello Frimkron

Welcome to Math Help Forum!This is in the right place - it's definitely a trig question.

Your equation can be re-arranged into the form:

, where , etc

You then need to let

When you expand the RHS and compare coefficients of and , you get:

which, if you square and add, and then divide one by the other, gives:

(Can you follow this OK?)

This then gives the equation in the form

from where you can solve for :

Can you fill in the details? (It will be complicated, given your initial equation, but you'll find that's the only method that will work.)

Grandad