The coordinates of the midpoint of ab are:

The answer should be 
c) (-2,5) 

It is the only point that falls on the line and when you count the units from (-7,2) to (-2,5) it is 5 units and when you count the distance from (3,8) to (-2,5) you also get 5 units, proving that (-2,5) is in the middle of the line.

The correct answer is C (-2,5)