The graph for the equation "y - 8 = -x" is a straight line that goes trough the points (0, 8) and (8, 0) since only one straight line passes through two distinct points.

You can confirm that if you plug "x=0" into "y - 8 = -x" and solve for "y". 

y - 8 = 0

y = 8

So, indeed (0, 8) is one point of the line.

The same for "x=8"

y - 8 =  - 8

y = 0

So, (8, 0) is another point of the line.