question

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°

what is the interior and exterior angles of a 21-agon

exterior angle $= \dfrac{360^{\circ}}{21} = 17.1428571429^{\circ}$

interior angle $= 180^{\circ} -$ exterior angle $= 180^{\circ} -17.1428571429^{\circ} = 162.857142857^{\circ}$