To find the reference angle for an angle in standard position,

1. Draw the angle in standard position, rotated from the right
side of the x-axis, counter-clockwise if the angle is positive
and clockwise if the angle is negative.

2. Calculate the acute angle not in standard position between
the terminal side and the x-axis.

So, for your peoblem:

1. We draw the angle of 210° in standard position:



2. We draw the reference angle, which is the acute angle which is not
in standard position between the terminal side of the 210° angle and 
the x-axis, shown by the red arc below.



The reference angle is the amount of rotation more than 180² the 210°
extends into the third quadrant.  So the reference angle is calculated
by subtracting 180° from 210°.  So the reference angle indicated by the 
the red arc is 210° - 180° = 30°.

So that's the answer.  The reference angle is always the acute angle
between the terminal side and the x-axis.



  

Edwin