Answer
$q=-2p+24$
Work Step by Step
A demand function expresses demand $q$ (the number of items demanded)
as a function of the unit price $p$ (the price per item).
If it is a linear function, it has form $q=mp+b$.
Given $(5,14)$ and $(3,18)$, two points on the graph, we find the slope:
$m=\displaystyle \frac{18-14}{3-5}=\frac{4}{-2}=-2$
So far, we have $q=-2p+b.$
Substitute the coordinates of $(5,14)$ and solve for b.
$14=-2(5)+b$
$14=-10+b\qquad/+10$
$24=b$
Thus,
$q=-2p+24$
