what is the measure of an exterior angle of a regular quadrilateral
-- Any "regular" polygon has all sides equal AND all angles equal.
-- The sum of the angles inside a 4-sided figure is 360° .
-- So it it's a regular figure, then each angle is 360°/4 = 90° .
-- The total angle around each vertex is 360° .
If 90° of them are inside, then the exterior angle is the remaining 270° .