Hello jrr

Welcome to Math Help Forum!It's not clear what the symbols are between the and the brackets, so I shall ignore them for now. (If it turns out to be a power, like the same method applies.)

Expand each of the cosine expressions on the LHS using , and then divide top and bottom by . Then solve the resulting equation for .

Here's the start:Can you now make the subject of this equation, and hence find ?

Grandad