There are three possible points.

1.  AB and AC are sides of the parallelogram and BC is a diagonal.
2.  AB and BC are sides of the parallelogram and AC is a diagonal.
3.  AC and BC are sides of the parallelogram and AB is a diagonal.

1. To get from A to C, we start at A, go down 2 units and right 4
units.  So we do the same thing starting at point B. We end up at
point D(5,3):
So we have this parallelogram: 

2. To get from C to A, we start at C, go up 2 units and left 4
units.  So we do the same thing starting at point B. We end up at
point D(-3,7):
So we have this parallelogram: 

3. To get from B to A, we start at B, go down 5 units and left 3
units.  So we do the same thing starting at point C. We end up at
point D(-1,-7):
So we have this parallelogram: 

Edwin