Answer:
m∠B = 21°
Explanation:
Congruent triangles have equal angles, equal sides, equal perimeters, and equal areas
Let us solve the question
∵ The two triangles ABC and DEF are congruent
∴ m∠B = m∠E
∵ m∠B = 3y
∵ m∠E = 21
→ Equate their measures
∴ 3y = 21
→ Divide both sides by 3
∵
=
∴ y = 7
→ Substitute y by 7 in the measures of angle B to find it
∵ m∠B = 3(7)
∴ m∠B = 21°