You are graphing rectangle abcd in the coordinate plane. the following are three of the vertices of the rectangle: a(-7, -7.5), b(-3, -7.5)c(-3,2.5). what are the coordinates of point d?

The midpoints of the two diagonals are the same, so ...
  D = A +C -B
  D = (-7-3+3, -7.5+2.5+7.5)
  D = (-7, 2.5)

Another way to look at it is that D will have the same x-coordinate as A and the same y-coordinate as C (in order to make each coordinate repeated twice).

The coordinates of point D are
  (-7, 2.5)