+600 I am assuming you want $\theta$ not $0$.
We have that $\cos\theta=\frac{-2\sqrt{2}}{4}=-\frac{\sqrt{2}}{2}$. Use the fact that $\cos^{-1}\left(-\frac{\sqrt{2}}{2}\right)=\frac{3\pi}{4}$ and the fact that the period of $\cos x$ is $2\pi$ to finish.
+600 Right, others besides just $\frac{3\pi}{4}$ work. I was just giving an example, because the rest follow from the period $2\pi$. In fact, the general solution is $\theta=2\pi n\pm \frac{3\pi}{4}$ for integer $n$.
