In a parallelogram, two diagonals of parallelogram bisects each other, which concludes that mid-point of both diagonals are the same. Midpoint of AC=(21−2​,21+8​) Let the coordinates of vertex D be (x,y)(2x+3​,2y+4​)=(21−2​,21+8​)x=−4 and y=5 The answer is option D.