ok well you already have the first, so here's the last:

(d/5) x 7

BECAUSE:

you need to find out the price for each individual book, so you would divide the price you paid (d) by the number of books you bought (5). Then you would multiply that answer by 7 to find out how much seven books would cost. Therefore:

(d/5) x 7

EXAMPLE:

Let's say I paid $20 for 5 books. I would need to find out how much I paid for each book, so I would divide 20 by 5 (which is, of course, 4). $4 is how much you paid per book, so therefore to find the price of seven books you would multiply seven by four, which equals 28

so basically:

(20/5) x 7 = 28

I really hope I explained that well

EDIT: WHAT!! You eliminated the answer?? That means it wasn't correct??

Well darn it then!! It works, I can't see how its wrong! O_o