If there is 9 slots per page, and 18 pages in the 54 page book are taken up by football cards, that leaves you with 36 pages left. Again, 9 slots per page, so you multiply 9 x 36 and get 324 slots available for baseball cards.
i put x<_324 / filled in circle 324/left
Nine time Nine times Two gets you the answer provided
the answer is 324