Answer
$q=-40p+2000$
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 $(1,1960)$ and $(5,1800)$, two pints on the graph, we find the slope:
$m=\displaystyle \frac{1800-1960}{5-1}=\frac{-160}{4}=-40.$
So far, we have $q=-40p+b.$
Substitute the coordinates of $(1,1960)$ and solve for b.
$1960=-40(1)+b\qquad/+40$
$2000=b$
Thus,
$q=-40p+2000$
