[tex]f(x) = 2 \sin(x) - x \\ f'(x) = (2 \sin(x) - x)' = (2 \sin(x)) ' - x' = \\ = 2' \sin(x) + 2 \sin(x) ' - 1 = 2 \cos(x) - 1 \\ f '(x) = 0 \: \: \: < = > 2 \cos(x) - 1 = 0 \\ \cos(x) = \frac{1}{2} \\ x = \arccos( \frac{1}{2} ) + 2\pi n = \frac{\pi}{6} + 2\pi n[/tex]