The missing reason in the proof is:
Transitive Property of Equality
The proof uses this property when it states that if \( m\angle A = m\angle E \) and \( m\angle E + m\angle C = 180° \), then it can conclude that \( m\angle A + m\angle C = 180° \) by substituting \( m\angle E \) for \( m\angle A \). This is an application of the Transitive Property.