It helps to draw out the problem on a piece of graph paper; if you do, you can see that the fourth point would be directly below (3,3) and directly to the right of (-1,-1), meaning that its coordinates would be (3,-1). A more algebraic method would be to subtract 4, the length of each side of the square, from the y-coordinate of (3,3) and to add 4 to the x-coordinate (-1,-1) to get the same point, (3,-1).
