Answer
$-0.25$ ft/day
Work Step by Step
From the given graph, apply the formula such as: $\dfrac{W(x_2)-W(x_1)}{x_2-x_1}$
Plug the given data in the above formula.
we have
$\dfrac{W(x_2)-W(x_1)}{x_2-x_1}=\dfrac{W(200)-W(100)}{200-100}$
This gives:
$\dfrac{W(200)-W(100)}{200-100}=\dfrac{50-75}{100}$
or, $\dfrac{W(200)-W(100)}{200-100}=-0.25$ ft/day