Answer
$\frac{3\pi}{4}, \frac{5\pi}{4}$
Work Step by Step
$3\sqrt 2 cos\theta+2=-1$,
$cos\theta=-\frac{\sqrt 2}{2}$,
$\theta=2k\pi+\frac{3\pi}{4}$ or $2k\pi+\frac{5\pi}{4}$ (k is an integer),
$\theta=\frac{3\pi}{4}, \frac{5\pi}{4}$ in $[0,2\pi)$.
