That is, the average rate of change of from 3 to 0 is 1. That is, over the interval [0,3], for every 1 unit change in x, there is a 1 unit change in the value of the function. Here is a graph of the function, the two points used, and the line connecting those two points.
The formula is as follows:
A(x) = (f(x) - f(a)) / x - a
I hope my answer has come to your help. God bless and have a nice day ahead!