Answer
$0,\frac{2\pi}{3}, \frac{4\pi}{3}$
Work Step by Step
$2cos^2\theta-cos\theta-1=0$,
$(2cos\theta+1)(cos\theta-1)=0$,
$cos\theta= -\frac{1}{2}$, thus $\theta=\frac{2\pi}{3}, \frac{4\pi}{3}$ or
$cos\theta= 1$, thus $\theta=0$ within $[0,2\pi)$.
