Hi - Needs some pointers on this question as can't find any examples like it in any books.

Find the general solutions to:

$\displaystyle \cos \left(7\theta \right) = \cos \left( 9\theta - \frac \pi{7} \right )$

Not really sure where to start with this at all! I know the general solution formulas but need a push in the right direction. Do I use the sum and difference identities, or compound identities to get to the double angle ones.?? really not sure so your help would be great.

Thanks, Felix