if you look at the picture, when
x = 4, y = 20,
and when
x = 8, y = 10
thus
[tex]\bf slope = m = \cfrac{rise}{run} \implies
\cfrac{ f(x_2) - f(x_1)}{ x_2 - x_1}\impliedby
\begin{array}{llll}
average~rate\\
of~change
\end{array}\\\\
-------------------------------\\\\
\begin{cases}
x_1=4\\
x_2=8
\end{cases}\implies \cfrac{f(8)-f(4)}{8-4}\implies \cfrac{10-20}{4}\implies \cfrac{-10}{4}\implies -\cfrac{5}{2}[/tex]
