Question 600309
To find the exterior angle of a regular polygon with 'n' sides, simply divide 360 by n
i.e. 360/n

In this case, the measure of the angle is 360/21
i.e. 17.1 (approximately)

To find the interior angle, subtract the exterior angle from 180, OR use the formula
Measure of interior angle of a regular polygon with 'n' sides = 180*{{{( 1-2/n )}}}

In this case, the interior angle = 180 - 17.1 = 162.9 (approximately)