To find the midpoints you would add the x values and divide by 2, then add the y values and divide by 2.
After drawing the parallelogram I found that the diagonals are EH and FG.
EH: (-3 + -2)/2, (1 + 6)/2 = (-5/2, 7/2)
FG: (-5 + 0)/2, (2 + 5)/2 = (-5/2, 7/2)
The midpoint of both diagonals is (-5/2, 7/2) OR (-2.5, 3.5)