According to the graph, we can check that, at t = 5.5 hours, the price of the commodity is approximately 11.75 $/lb and, at t = 4.5 hours, the price is approximately 13.75 $/lb. Therefore, we can approximate f'(5) as the slope of the segment connecting these two points as follows:
[tex]f^{\prime}(5)\cong\frac{11.75-13.75}{5.5-4.5}=-2\frac{\text{ \$}}{lb\cdot h}[/tex]
