Follow the following steps to work out with this problem:-
First draw the parallelogram with AB//CD and AD//BC.
Draw the diagonals AC and BD to intersect at O.
We know that diagonals of a parallelogram bisect each other at the point of intersection.
Therefore O is the midpoint of AC and BD.
Now find the co-ordinates of the point O by applying section formula on the diagonals AC (ratio in which O divides AC and BD is 1:1)
The co-ordinates come out to be O[(x1+x3)/2],[(y1+y3)/2].
Let us assume that co-ordinates of D is D(a,b).
Apply section formula on line segment BD and again find the co-ordinates of point O.
(Ratio is 1:1).
The co-ordinates come out to be O[(x2+a)/2],[(y2+b)/2].
Since the co-ordinates are of the same point O, they are equal.
Equate x coordinate and y coordinate separately.
The answer that follows is
D(a,b)=D(x1+x3-x2 , y1+y3-y2).