Bookworm:
y = 14x
Star Readers:
y = 80 + 8x
IF you want to see when you pay the same amount, then we set up a system of equations:
14x = 80 + 8x
6x = 80
x = 40/3, so when you buy 13 or less books is the Bookworm bookstore better. Star Readers is better when you have 14 or more books.