The ex- suffix often correlates a word to mean "outside", while the in- suffix often correlates a word to mean "inside". An exterior angle of a polygon would mean "an angle outside of a polygon". An interior angle of a polygon would mean "an angle inside of a polygon". Three exterior angles of this polygon would be angle B, angle D, and angle A. This is because these angles are outside of the polygon due to the extending lines from the shape. Two interior angles of this polygon would be angle 6 and angle 8 (explanation was given when I first began answering this question). Angle 9 would be exterior since it is outside of the polygon. Two exterior angles of the polygon that are congruent are angle D and angle 9, since they are both 90 degrees (right angles).
