Find the depth of the river at 9 hours and 18 hours by seeing where the line is on the Y axis.
At 9 hours the river is 18 feet deep
At 18 hours the river is 21 feet deep.
The average rate of change is the change in Y over the change in X
The Y is the depth and the X is the time:
(21-18) / (18-9) = 3/9 = 1/3 = 0.33
The average rate of change is 0.3 foot per hour. ( Rounded to nearest tenth)