
Answer:
(-1,-1)
The square needs to be all side with the same value when you graph the vertices on x-y plot, you obtain (x is the dot that correspond a vertice and y the forth vertice )
              x    2 |             x
                   1 |
                    |                 Â
   -2          -1     |      1        2
              y     -1|            x
                   -2 |
If you draw a line between vertices the value it will be 2 + Â (-1) so the forth vertice has to be (-1,-1)