Statement 1.
Reason 1. Alternate interior angles formed when two parallel lines are cut by a transversal are congruent.
Statement 2.
Reason 2. Reflexive congruence of segments (a segment is congruent to itself).
Statement 3.
Reason 3. SAS congruence (using our two congruences above and the given congruence).
Statement 4.
Reason 4. Corresponding parts of congruent triangles are congruent.
Statement 5.
Reason 5. If the alternate interior angles formed when two lines are cut by a transversal are congruent, then the lines are parallel.
Statement 6.
is a parallelogram.
Reason 6. The quadrilateral satisfies the definition of a parallelogram: a quadrilateral where both pairs of opposite sides are parallel.