Answer
(a) $30^\circ$
(b) $80^\circ$
(c) $ 90^\circ$
Work Step by Step
The angle $\theta$ we are looking for is the positive angle of the terminal line with the x-axis:
(a) $390^\circ=360^\circ+30^\circ$, thus $\theta=30^\circ$
(b) For $-80^\circ$, $\theta=80^\circ$
(c) $810^\circ=720^\circ+90^\circ$, thus $\theta=90^\circ$
