Answer:
∠ C = 100°
Explanation:
AC and BC are congruent sides of the triangle.
the triangle is therefore isosceles with base angles congruent, that is
∠ A = ∠ B = 40°
the sum of the 3 angles in a triangle = 180° , then
∠ A + ∠ B + ∠ C = 180° , that is
40° + 40° + ∠ C = 180°
80° + ∠ C = 180° ( subtract 80° from both sides )
∠ C = 100°