This is part of a bigger question to do with polar coordinates - I have obtained a derivative and equated it to zero in order to indicate a stationary point and this is what I need to solve:

$\displaystyle 2cos(2x)cos(x) - sin(2x)sin(x) = 0$

Which of the double/half/compound angle formulas do I need to use here? I always seem to forget in questions like these.

Thanks if you can help me :]