The displacement from point B to point A is:
So the column matrix representing the displacement is:
e = (-5, -2)T
When the points are visited in the opposite order, the displacement vector points in the opposite direction. In the column matrix each element is -1 times the old value.
The displacement from A to B is different from the displacement from B to A. Think of displacement as "directions on how to walk from one point to another." So, if you are standing on point A and wish to get to point B, the displacement (3, 1)T says "walk 3 units in the positive X direction, then walk 1 unit in the positive Y direction."
Of course, to get from point B to point A you need different directions: the displacement (-3, -1)T says "walk 3 units in the negative X direction, then walk 1 unit in the negative Y direction," which puts you back on point A.
The displacement from point Start to point Finish ==
(Finish x - Start x , Finish y - Start y)T