The midpoint of a segment or a diagonal refers to the point in the middle of an segment. This can be find by using the formula, M=(x1+x2/2,y1+y2/2).
On this exercise is given that a parallelogram contains the points G and J with coordinates (4,7) and (-1,5) receptively. It is asked to find the midpoint of the diagonal, which is a segment, to do that you should substitute the given points into the midpoint formula.
M=(x1+x2/2,y1+y2/2)
M=(4+-1/2,7+5/2)
M=(3/2,12/2)
M=(3/2,6) or (1.5,6)
The midpoint of the diagonal is the coordinate (3/2,6) or (1.5,6).