Remember that
A relationship between two variables, x, and y, represent a proportional variation if it can be expressed in the form y=kx
In a proportional relationship, the constant of proportionality k is equal to the slope m of the line and the line passes through the origin
In this problem, to find out the slope of the line, we need only one point
so
k=y/x
we take the point
(4,6)
k=6/4
simplify
k=3/2
k=1.5
that means -----> $1.5 per every book sold