Answer: -40
Explanation:
Rate of change is calculated as the slope. The formula for Slope is:
S =

The two points we have are when x = 3 and 6.
the points are (3, 120) and (6, 0), as we can see.
plugging into the slope formula:
S =

S = 120/-3
S = -40
Which hopefully makes sense, because the slope is negative, (the graph is falling).