Geometry Help! INDIRECT proofs!!?

1. Assume that line a is parallel to line b. Since line a is parrallel to line b and cut by the transversal t, m<1 is congruent to m<3. By vertically congruent angles, m<3 is congruent to m<2. So by the transivity property, m<1 is congruent to m<2. But, m<1 > m<2, a contradiction.