Hello. Could you please help with my math homework...?

An online bookseller gives the following information for their shipping information: "The shipping and handing charge per order is $4, plus $1.50 per book."

Let x = the number of books that someone orders.
Let S(x) = the shipping charge for the order in dollars.

What is a function formula for S(x)?

S(x) = 4.00 + 1.50x

Now that wasn't so hard, eh?