If the cost for 6 books is $13.50 then to get the price of just one book you must divide 13.50 by 6.
The answer should be $2.25 per book purchased.
To find the price of two books you can multiply 2.25 by 2 OR you can add 2.25 and 2.25 together.
The answer should be $4.50 for two books.