Answer
$30^o$
Work Step by Step
RECALL:
The following are the means on how to find the reference angle of an angle $\theta$ based on its position:
(1) Quadrant I: $\theta$ (itself)
(2) Quadrant II: $180^o-\theta$
(3) Quadrant III: $\theta - 180^o$
(4) Quadrant IV: $360^o-\theta$
$210^o$ is in Quadrant III.
Thus, using forrmula (3) above gives:
reference angle of $210^o$ = $210^o-180^o=30^o$
