Remember rules with angles when parallels are present (alternate interior/exterior angles, corresponding angles, same side interior/exterior angles, etc.)
Statements Reasons
∠3 = ∠5 m || n, alternate interior angles
∠5 = ∠13 a || b, corresponding angles
∠3 = ∠13 substitution property: ∠3 =∠5 and ∠5 =∠13