You can put this solution on YOUR website!
If I am given three points and ask to find a fourth so that they are vertices of a parallelogram, how do I find the fourth?
Plot the three points so you can see what you are doing.
The three points will determine two sides of the parallelogram
if you choose the proper pair of points for each line.
Now you should see approximately where the 4th point has to be
You need to find the equation of two lines; each of the lines
is parallel to one of the original sides and passes thru the
proper known point.
Once you get those two equations you need to find the intersection
of those two lines to determine the 4th vertex of the parallelogram.