#1**+2 **

The sum of all the interior angles = 180° * (21 - 2) = 3420°

If this is a regular polygon, then all of 21 the interior angles have the same measure.

So the measure of each interior angle = 3420° / 21 ≈ 162.86°

And the measure of each exterior angle ≈ 180° - 162.86° = 17.14°

hectictar
Nov 2, 2017