What is the fourth vertex of (5,,5) (2,-3)

It can be anywhere on the coordinate plane. 

You haven't specified any particular shape for the
resulting figure, so there are no constraints on the
location of another vertex.

The 3 points you did specify give us 2 sides of the figure.
The sides are not perpendicular.

IF the figure is supposed to be a parallelogram, then
the fourth vertex is  (-3, 2) .  But there's no reason that
it has to be there, because you haven't said that the
figure must have any particular shape.

(-3,2) I think, because (5,0) is switched to make (0,5) so switch (2,-3) to get that.