HELP!!!!!!!!數學幫幫手!!!!

Let(h, k) be the coordinates of R.

PR and QS are the diagonals of the parallelogram PQRS.
The mid-point of QS = ((a - b)/2, (a + b)/2)
The diagonals of a parallelogram bisect each other.
Hence, ((a - b)/2, (a + b)/2) is also the mid-point of PR.

Calculate the mid-point of PR :
(0 + h)/2 = (a - b)/2 and (0 + k)/2 = (a + b)/2
h = a - b and k = a + b
Hence, the coordinates of R = (a - b, a + b) ...... The answer is D.