TeX source:

\begin{aligned} \text { Average rate of change } &=\dfrac{f(4)-f(2)}{4-2} \\ &=\dfrac{\dfrac{63}{4}-\dfrac{7}{2}}{4-2} \\ &=\dfrac{\dfrac{49}{4}}{2} \\ &=\dfrac{49}{8} \end{aligned}