Looking at the diagram, there is a transversal cutting across lines m ans n. The angles formed are congruent because they are 90 degrees. We can see that the angles are in similar positions and in the same side of the transversal. This means that they are corresponding angles.
Recall the corresponding angles postulate which states that if corresponding angles are congruent when two lines are crossed by a transversal, then the two lines crossed by the transversal are parallel
Thus,
line l is parallel to line m
The correponding angles converse prove that they are parallel