m∠1 = 76° Given.
m∠1 ≅ m∠3 Vertical Angle theorem.
m∠3 = 76° CPCTC
m∠1 + m∠2 = 180° (supplementary angles)
m∠3 + m∠2 = 180° (supplementary angles)
m∠1 & m∠3 ∴ Congruent (congruent supplements theorem)
m∠2 + m∠3 = 180° (supplementary angles)
m∠4 + m∠3 = 180° (supplementary angles)
m∠2 & m∠4 ∴ Congruent (congruent supplements theorem)
∴ IF m∠1 = 76° (Given)
THEN:
m∠2 = 104° (Supplementary angles)
m∠3 = 76° (Definition of vertical angles)
m∠4 = 104° (Supplementary angles)