Answer
$q=-4500p+41,500$
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 $(3,28000)$ and $(5,19000)$, two points on the graph, we find the slope:
$m=\displaystyle \frac{19000-28000}{5-3}=\frac{-9000}{2}=-4500$
So far, we have $q=-4500p+b.$
Substitute the coordinates of $(5,19000)$ and solve for b.
$19,000=-4500(5)+b$
$19,000=-22,500+b\qquad/+22,500$
$41,500=b$
Thus,
$q=-4500p+41,500$
