Ok so the equation is

$\displaystyle f(x) = cos^2(x) - 2sin(x)$

0 < or equal to x < or equal to 2pi

I get the derivative which is

$\displaystyle f'(x) = -sin^2(x) - 2cos(x)$

I was never good with Trigonometric identities so I need help with finding the critical numbers.