As you're looking for a square, this means that the two sets of coordinates have to be the same ( meaning that the two coordinates at the top and the bottom of the square have to be the same, eg 4 and 4 and 9 and 9). This means that one of this square will be 8 and 8, and 3 and 3 (as you've got one three, so the other one must be the same). The same rule applies to the sides of the square. In this case, you've got 2 and 2 and 7, therefore, meaning that the other side has to be 7. As you've got a square, this means that they must be two sets of opposite coordinates.
Therefore, the missing coordinate that you are looking for is (3,7)
Hope this helps :)
