19.
To find this you first have to find the f(x) values for x = 5 and x = 8
x = 5
f(x) = 5^2 + 6(5) + 11
f(x) = 25 + 30 + 11
f(x) = 66
x = 8
f(x) = 8^2 + 6(8) + 11
f(x) = 64 + 48 + 11
f(x) = 123
Then you subtract the two from each other to see how much it has changed in those 3 numbers.
123 - 66 = 57.
Then you take that answer and divide it by the change in x (which is 3)
57/3 = 19
