Answer:
The expression showing how many more meters Mathieu ran than Edgar ran during that time is
.
Step-by-step explanation:
Edgar ran 'e' meters per second, and Mathieu ran 'm' meters per second. The boys ran for 't' seconds.
The expression
describes how many more meters Mathieu ran than Edgar ran during that time.
We can also use the expression
represent the same quantity.
We know that,

Distance covered by Edgar = te
Distance covered by Mathieu = tm
Difference in distance 
The expression showing how many more meters Mathieu ran than Edgar ran during that time is
.