Hey, I'm back with another homework hand-in problem that I can't figure out how to do and was hoping someone could help me out here. I need to find all the points of horizontal tangent lines of

y = x + Cos(2x) where 0<= x <= 2pi

Now I was going to use 0 and 2pi as the end points, find the derivative of the problem, and then solve for x and set it = to 0. But... the derivative is

1 + -2Sin(2x) and it kills off the only x I could work with...