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 |
|
-2 -1 | 1 2
y -1| x
-2 |
If you draw a line between vertices the value it will be 2 + (-1) so the forth vertice has to be (-1,-1)