Line l bisects xy at m and divides xy into two equal parts. name a pair of congruent segments.

The term congruent means exactly equal. Congruent line segments means exactly equal line segments (in length of course).

Since the line L bisects XY at m and divides it into two EQUAL parts, therefore these two parts are said to be congruent.

Based on this, the pair of congruent segments would be XM and MY. 

What's the denominator