First we need to see in which quadrant 11pi/6 lies And it lies in fourth quadrant. And to find the reference angle in the fourth quadrant, we need to subtract the given angle from 2 pi . For e.g if the angle is 5pi/3, then the reference angle is 2pi - (5pi/3) = pi/3 .
So for the given question, reference angle of 11pi/6 is
And that's the required reference angle .