Answer
$\frac{2\pi}{3}, \frac{4\pi}{3}$
Work Step by Step
Given $cos(s)=-\frac{1}{2}$, we can find a reference angle as $s_0=cos^{-1}(\frac{1}{2})=\frac{\pi}{3}$. In $[0,2\pi)$, there are two solution angles $s=\pi-\frac{\pi}{3}, \pi+\frac{\pi}{3}$ or $s=\frac{2\pi}{3}, \frac{4\pi}{3}$
