TeX source:

\begin{aligned} \text { Average rate of change } &=\dfrac{\text { Change in output }}{\text { Change in input }} \\ &=\dfrac{\Delta y}{\Delta x} \\ &=\dfrac{y_{2}-y_{1}}{x_{2}-x_{1}} \\ &=\dfrac{f\left(x_{2}\right)-f\left(x_{1}\right)}{x_{2}-x_{1}} \end{aligned}