So, this is the answers you get from plugging in x = (7 through 10)
x = 7 ... 24
x = 8 ... 36
x = 9 ... 50
x = 10 ... 66
Then you get the rate of changes...
66 - 50 = 16
50 - 36 = 14
36 - 24 = 12
So the average of 16, 14 and 12 (those values added together and then that total divided by the number of values, which is 3) is 14
The average rate of change is 14
