The given points have the same Y value.
Find the distance between the X values -3 to 1 is 4 units.
All sides of a square are the same length so the other points must be 4 units away.
Add 4 units to the Y values:
(-3,2) = (-3,(2+4)) = (-3,6)
(1,2) = (1,(2+4)) = (1,6)
Two other points to make a square could be (-3,6) and (1,6)