The ordered pair for C would be (3,-7)
The ordered pair for D would be (3,5)
My explanation would be that i counted that line segment AB is 12 units long. Then i doubled 12 (Because 12 would be on both sides of the rectangle) and had the equation 40-24 this equals 16. Finally you divide 16 by 2, so you can have the rest of the perimeter split up equally to equal 40 units.