If you plot these points, you can probably see where the missing square corner is. But you might also notice that a square's sides are equal length. Between the first two coordinate pairs, the only thing that changes is the y value, from 5 to 2, or a change of 3. Between the 2nd two coordinate pairs, only the x value changes, from -5 to -2, or a change of 3. So it looks like the sides are 3 units long. The fourth coordinate should be 3 units away on the x coordinate from one "neighbor" and 3 units away on the y coordinate from its other neighbor. But it really would help to sketch it out so you don't get mixed up.
(-2, 5)