Remember that when parallel lines are cut by a transversal, there are pairs of angles formed. Some of those pairs are congruent while the others are supplementary.
In the given diagram, the angles x and a are corresponding angles, so they are congruent. Then:
[tex]m\angle x=m\angle a[/tex]
This only happens when there is a pair of parallel lines. Otherwise, we would have to know the measure of the angle a in order to compare it to the angle x.
Then, the comparison cannot be made if it is uncertain whether or not the lines are parallel.
