Answer
$\frac{3\pi}{4},\frac{5\pi}{4}$
Work Step by Step
Using trigonometric identities, we have
$3\sqrt 2cos\theta+2=-1 \longrightarrow cos\theta=-\frac{\sqrt 2}{2} \longrightarrow \theta=\pi-\frac{\pi}{4},\pi+\frac{\pi}{4} \longrightarrow \theta=\frac{3\pi}{4},\frac{5\pi}{4}$
