The average rate of change of a function f(x) over some interval [a, b] is the difference quotient,
![(f(b)-f(a))/(b-a)](https://img.qammunity.org/2023/formulas/mathematics/high-school/s8o0h098jwm2d2dh8wf7dbquzgg5mr8lyy.png)
which corresponds to the slope of the line connecting the points (a, f(a)) and (b, f(b)) in the graph of f(x).
Given f(x) = 3x² - x³ (correct me if I'm wrong, the exponents look cut off in your screenshot), the average rate of change on [1, 5] is
![(f(5)-f(1))/(5-1)=\frac{(3\cdot5^2-5^3)-(3\cdot1^2-1^3)}4 = \frac{75-125-3+1}4 = \boxed{-13}](https://img.qammunity.org/2023/formulas/mathematics/high-school/2whpds00jw9yfghkqnoxug302dq1i4rc9a.png)