If given 3 vertices of parallelogram, how do I find coordinates of 4th vertex?

Question

Vertices P(0,0), Q(6,0), R(2,5). Find possible coordinates of S.

Answer 1

length av base=6, so top line gotta be 6

2+6=8...so rite end av top=(8,5)

Answer 2

First, construct the diagonals of the parallelogram, then find the coordinates of the point where they bisect. In this case it is (1, 5/2). Then use this point to find the point S. In this case, S(-4, 5).