SOLUTION: If three corners of a parallelogram are (1,1), (4,2) and (1,3) then the fourth corner is........

This problem has three different solutions:



Here they are:

    

The points circled in blue are the solutions.  I'll let you get the
coordinates of those three points.

If you were to put all three solutions on the same graph, you'd get 
what's below, which is quite interesting that you get a big triangle
with a small triangle inside whose vertices are the three given points,
and they are the midpoints of the sides of the big triangle:



Edwin