EXPLANATION
Coterminal angle in [0, 360°) range is 261°, and it's located in the third quadrant.
We need to use a modulo operation, divide the given angle by the 360° and check what the remainder is.
621 mod 360 = 261°
In the case of the third quadrant, the referent angle is given by subtracting the coterminal angle to 180 degrees as shown as follows:
261 - 180 = 81 degrees [Reference angle]