The complex number given to me was 2-$\sqrt{3}$i.

I'm supposed to turn this into the polar form, and so I solved for r using r = $\sqrt{a^2+b^2}$, getting $\sqrt{7}$. When I solved for theta using tan$\theta$ = $\frac ba$ , I got tan$\theta$ = -$\sqrt{3}$ over 2. I know that I would get the inverse of that and then add it to 360. The degree would be around 319.1 degrees.

My question is why would I have to add additional degrees to it ? Another problem was similar to this but I added it to 180 instead but I didn't know why. Why do I add it and when is it a good time to use it ? How would I know which one is the appropriate one to use ?