Given: lines a and b are parallel and line c is a transversal.
prove: angle2 is supplementary to angle8

horizontal and parallel lines a and b are cut by transversal c. on line a where it intersects line c, 4 angles are created. labeled clockwise, from uppercase left, the angles are: 1, 2, 4, 3. on line b where it intersects line c, 4 angles are created. labeled clockwise, from uppercase left, the angles are: 5, 6, 8, 7.
what is the missing reason in the proof?

statement reason
1. a || b, is a transv 1. given
2. ∠6 ≅ ∠2 2. ?
3. m∠6 = m∠2 3. def. of congruent
4. ∠6 is supp. to ∠8 4. def. of linear pair
5. ∠2 is supp. to ∠8 5. congruent supplements theorem
corresponding angles theorem
alternate interior angles theorem
vertical angles theorem
alternate exterior angles theorem