I know I am missing something simple. $\displaystyle tan\theta=\frac{-1}{2}$ Having a hard time figuring out what $\displaystyle \theta$ is.
This one doesn't come out nice, so you're stuck with using technology. From my calculator I get that $\displaystyle \theta = tan^{-1} \left ( - \frac{1}{2} \right ) \approx -26.5651^o$.