Answer: -1
Explanation:
An easy way to figure out the slop of a linear graph is the formula:
(y1-y2)/(x1-x2)
In order to use this, find two spots on the line that run exactly through a point, in this example (0,20) and (20,0).
Plug in the xs and ys into the formula, which gets you
(20-0) / (0-20)
When simplified, the result is -1 and that is your slope