Question

Find the sum of the measures of the interior angles of a 21-gon.

Answer 1

Well you have to use the equation     (n-2(180))  where n represents the number of sides  

so-    21 - 2 = 19      then multiply    19 x 180 = 3420

so the sum of all the interior angles in a 21-gon is equal to    3420 degrees

Answer 2

We know that
Sum of the measures of the interior angles of a polygon of n sides is equal to

$(n-2)*180$

where
n is the number of sides

in this problem
$n=21 \\ (21-2)*180=3420$°

Hence

sum of the measures of the interior angles of a 21 sided polygon is
$3420$°