Final answer:
To calculate the cost of y books, start with the cost of the first copy and then add the cost of the additional copies using the given information.
Step-by-step explanation:
The cost of y books can be calculated using the given information. For the first copy of the book, the cost is $15. For each additional copy, the cost is $12. So, if you need y additional copies, the cost would be $12 multiplied by y, which can be written as 12y. Adding the cost of the first copy, the expression that represents the total cost of y books is 15 + 12y.