You can put this solution on YOUR website! y=-x+8 (red line)
x+y=7
y=-x+7 (green line) (graph 300x200 pixels, x from -6 to 10, y from -10 to 10, of TWO functions -x +8 and -x +7).
As you can see these 2 lines are parallel; therefore there is no unique solution for x & y.