How to find the measure of an exterior angle given the measure of the interior angle?

Exterior angles are always supplementary to the interior angle, because to get an exterior angle you have to keep the line going from one interior angle. That means that 

(measure of interior angle)+(measure of exterior angle)=180 degrees

You can change the form of the equation and get

(measure of exterior angle)=180 degrees - (measure of interior angle)

So, to find the exterior angle, just subtract the interior angle from 180.

Hope this helps!