Answer
$q=-50p+4200$
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 $(5,3950)$ and $(10,3700)$, two points on the graph, we find the slope:
$m=\displaystyle \frac{3700-3950}{10-5}=\frac{-250}{5}=-50.$
So far, we have $q=-50p+b.$
Substitute the coordinates of $(5,3950)$ and solve for b.
$3950=-50(5)+b$
$3950=-250+b\qquad/+250$
$4200=b$
Thus,
$q=-50p+4200$
