Average rate of change ... of what?
Given some continuous function [tex]f(x)[/tex] and some interval [tex][a,b][/tex], its average rate of change over the interval is
[tex]\dfrac{f(b)-f(a)}{b-a}[/tex]
Without knowing what your function is exactly, I can only give a symbolic answer,
[tex]\dfrac{f(18)-f(0)}{18}[/tex]
