Since you know the triangles are congruent/equal, you know that:
m∠A and m∠X are congruent and have the same angle, and so does:
m∠B and m∠Y
m∠C and m∠Z
A triangle is 180°. (the 3 angles have to add up to 180) To find m∠B, you can do this:
m∠A + m∠B + m∠C = 180°
21° + m∠B + 35° = 180° Subtract 21 and 35 on both sides
m∠B = 124° your answer is C
