Okay - so 12pi/6 = 360. We need to make 31pi/6 between 0pi/6 and 12pi/6 in order to obtain the reference angle. So let's do that.
31pi/6 - 12pi/6 = 19pi/6
19pi/6 - 12pi/6 = 7pi/6
The reference angle in radians is 7pi/6, but it wants degrees - so we convert to degrees using 180/pi.
7pi/6 * 180/pi = 210 degrees
