y = 2cosx + cos(2x)

I guess I'm having trouble with the trig on this (i suck at trig).

The 1st derivative would be -2sinx -2sin2x ... then I try to set that equal to zero and ... yeah. =/ I think sin2x = 2(cosx)(sinx) but then I don't know how to solve for that equal to zero. Any help would be appreciated. Thank you guys.