Question 277205
y=-x+8 (red line)
x+y=7
y=-x+7 (green line)
{{{ graph( 300, 200, -6, 10, -10, 10, -x +8, -x +7) }}} (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.