For this case we have the following variable:
x = number of books
We write a function of the form:
y = mx + b
Where,
m = 2.09 $ / book
b = 349 $
Substituting values:
a (x) = 2.09x + 349
Answer:
A function that models a (x) the average cost per book of printing x books is:
a (x) = 2.09x + 349