the sum of all interior angles in a polygon is 180(n - 2), where n = number of sides.
well, this is a quadrilateral, so it has 4 sides so it has a total of interior angles of 180(4-2) = 360°, since this is a kite, then one pair of opposite angles are congruent, namely ∡C = ∡A.
360° - 45° - 100° = 215°, and ∡C and ∡A split that evenly, so ∡C = 107.5°.
