Answer
$\theta=120°$
$\theta=240°$
Work Step by Step
Let $\theta'$ be a refernce angle such that $\theta'$ lies in the first quadrant and:
$cos~\theta'=\frac{\sqrt 3}{2}$
Then:
$\theta'=60°$
If $cos~\theta\lt0$, then $\theta$ lies in the second or in the third quadrant:
$\theta=180°-\theta'=180°-60°=120°$
or
$\theta=180°+\theta'=180°+60°=240 °$
