Answer:
Ana  spent $17  per book on average.
Juan  spent $14.78  per book on average.
y = $15 + 17x--------- equation 1
y= $10 + 14.78 x------------- equation 2
Step-by-step explanation:
Ana has to pay a yearly fee of $15 to join. 
She spends a total of $525 for 30 books. 
The amount spent on books = Total amount - yearly fee
                                                  = $525- $15= $510
Price per book on the average = $510/30= $ 17
Ana  spent $17  per book on average.
Juan has to pay a yearly fee of $10 to join.
 He spends a total of $350 for 23 books.
The amount spent on books = Total amount - yearly fee
                                                  = $350- $10= $340
Price per book on the average = $340/23= $ 14.78≅ $15
Juan  spent $14.78  per book on average.
The function can be established if y gives the total cost and x gives the number of books
y = $15 + 17x--------- equation 1
y= $10 + 14.78 x------------- equation 2
Juan has a better deal because the yearly fee is less also the price per book is less.