Question
Let S be the set of all values of $\theta \in[-\pi, \pi]$ for which the system of linear equations
$x+y+\sqrt{3} z=0$
$-x+(\tan \theta) y+\sqrt{7} z=0$
$x+y+(\tan \theta) z=0$
has non-trivial solution. Then $\frac{120}{\pi} \sum_\limits{\theta \in \mathrm{s}} \theta$ is equal to :