In order to find the midpoint of a line segment you must find the point half way between the two points by simply dividing the sum of the coordinates by 2.  So for points a(x1,y1) and b(x2,y2) the midpoint is   



Midpoint Formula

         ( x1,y1)     (x2,y2)
        a(2,3) and b(4,1) has a midpoint

= (3,2)