Answer
$D(q)=-0.5q+72.50$
Work Step by Step
A linear function will have the form
$D(q)=p=mq+b$
The graph contains points
( demanded number of pills, price).
Using the points (50, 47.50) and (80, 32.50), we find the slope:
$m=\displaystyle \frac{47.50-32.50}{50-80} =\displaystyle \frac{15}{-30}=\frac{-1}{2}=-0.5$.
The point=slope form:
$p-47.50=-0.5(q-50)$
$p-47.50=-0.5q+25\qquad/+47.50$
$p=-0.5q+72.50$
or
$D(q)=-0.5q+72.50$
