Average Rate of Change

The formula for the average rate of change of a function $f$ between two values a and $b$ is given by

Average Rate $of$ Change $=\frac{f(b)-f(a)}{b-a}.$

Create a Python function that takes in a mathematical function, e$.$g$.,f(x)=3x^2$ and two numbers a and $b$

and returns the average rate of change of the function between a and $b.$