Chapter 5 - Relationships Within Triangles - 5-5 Indirect Proof - Practice and Problem-Solving Exercises - Page 321: 20

If a transversal intersects two parallel lines, then alternate exterior angles are not congruent.

For the first step in an indirect proof, we have to negate the "then" part of the statement. So we can write that if a transversal intersects two parallel lines, then alternate exterior angles are not congruent.