The ray of light, emitting from point M, reflects the line AB at point C ∈ AB and then goes through point N so that m∠ACM=m∠BCN. Prove that the bisector of the ∠MCN is perpendicular to line AB.

Sanderson is having trouble with his assignment. his shown work is as follows: − 3 7 7 3 = − 3 7 × 7 3 = − 21 21 = −1 however, his answer does not match the answer that his teacher gives him. complete the description of sanderson's mistake. find the correct answer.