Here is a similar, but not identical example worked out for you, based on the folowing graph:
\begin{align*}
\text{Average rate of change of } f \text{ over } [1996, 1998] &= \frac{f(1998)-f(1996)}{1998 - 1996}\\
&= \frac{ 1.1 - 1.3 }{2} = 0.1
\end{align*}
Since this is negative, west coast exports to Asia were decreasing at an average rate of 0.1 million containers per year over the period 1996-1998.
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