You can put this solution on YOUR website! x-y=-3 & x-y=4 do not have unique solutions:
x=y-3
(y-3)-y=4
y-3-y=4
-3=4 not a solution.
even if you attempt to graph these 2 equations there is no solution: (graph 300x200 pixels, x from -6 to 5, y from -10 to 10, of TWO functions y = x +3 and y = x -4).