Explanation 1
m∠1 + 50 = 180
m∠3 + 50 = 180
(Since supplementary angles' sum is equivalent to 180 degrees)
Subtract both sides by 50 for both equations:
m∠1 = 180- 50
m∠3 = 180- 50
So
m∠1 = 130
m∠3 = 130
Explanation 2
∠1 and ∠3 are congruent because of the Congruent Supplements Theorem. The theorem states that if two angles have congruent supplements (in this case, 50 degrees), then the two angles are congruent.