how to find atan (-9/(3*sqrt 3)) in radius
\(atan(\frac{-9}{3\sqrt3})=atan(\frac{-3}{\sqrt3})=atan(-\sqrt3)\\ =atan(-\sqrt3)=atan(tan(-60^\circ))=atan(tan(-\frac{\pi}{3}^c))=-\frac{\pi}{3}\; radians\)
