By using the given information that angle 7 is congruent to angle 3, we can prove that line a is parallel to line b.
To prove that line a is parallel to line b, we need to use the given information that angle 7 is congruent to angle 3.
The angles formed by a transversal cutting two parallel lines are congruent.
Since angle 7 is congruent to angle 3, this implies that line a is parallel to line b.