Given: ∠3 = ∠4
1) ∠1 = ∠4 because of vertical angles
2) ∠4 = ∠3, given
3) So ∠1 = ∠4 = ∠3, so ∠1 = ∠3, because of transitive property
2) ∠3 + ∠2 = 180 degrees because definition of a straight line
3) ∠1 + ∠2 = 180 degrees, using substitution
4) Since ∠1 + ∠2 adds up to 180 degrees, that makes them supplementary angles.