Hello!
The answers are:
Why?
For the first question, if there is no printed book, the variable x will be 0, so:

The cost to get started, meaning that there is not any printed book, is $1200
For the second question, to determine the change in the total cost for each printed book, let's assume that there is just one book printed, so:

So, the change for each printed book is $15(+) because:

Have a nice day!