Answer:
180°
Step-by-step explanation:
A "3-gon" is a triangle. The sum of the measures of its interior angles is 180°.
_____
You can make use of the formula for the sum of interior angles for an n-gon:
angle sum = 180°·(n -2) = 180°·(3 -2) = 180°
or, you can translate the exterior angles so they all have the same vertex and realize they sum to 360°. Then if the interior angles are A, B, C, you have ...
(180 -A) +(180 -B) +(180 -C) = 360
3·180 -(A +B +C) = 2·180 . . . . . collect terms
180 = A + B + C . . . . . . . . . . . . subtract 2·180 and rearrange
