Choose any two points on the line and find the rate of change between them.

We will the points at (1, 2) and (3, 6).

(1, 2)1 pound, 2 sales.

(3, 6)3 pounds, 6 sales.

Rate of change = .

The sales changes from 6$ to 2$ and the pounds changes from 3p to 1p.

Rate of change =

Rate of change =

Rate of change = .               (Divide: )

So, the rate of change is , or a increase of 2$ per 1 pound.

The rate of change is .