the complete question in the attached figure
we know that
pi/6-------> 180°/6--------> 30°--------> I quadrant
in the II quadrant--------------> 180-30-----> 150°
in the III quadrant------------> 180+30-----> 210°
in the IV quadrant---------> 360-30-----> 330°
in the I quadrant too-----> 360+30----> 390°
so
case 1) 8pi/6-------> 8*180/6----> 240°----> is not reference
case 2) 5pi/6-------> 5*180/6----> 150°----> is reference
case 3) 3pi/6-------> 3*180/6----> 90°----> is not reference
case 4) 8pi/6-------> 13*180/6----> 390°----> is reference
