The sum of the interior angles of a regular polygon with n sides is 180(n-2).
So, each interior angle has measure 180(n-2) / n.
Each exterior angle is the supplement to an interior angle.
So, the measure of an exterior angle is:
180 - 180(n-2) / n = [180n - 180n + 360] / n
= 360/n
Sum of exterior angles = n(360 / n) = 360.