With Gulliver's Travels in the first spot and A Tale if Two Cities in the last spot, there are 6 empty spots in between. Any of 6 books can go in the second spot, and then there 5 books left to pick from for the third place. That leaves 4 books to choose from for the next space and then 3 and then 2 and then one last book left to fill in (in front of Tale of Two Cities) since there's so many choices, we actually multiply to calculate how many different versions of the shelf we can get.
6•5•4•3•2•1 is 720 ways this can happen.
The mathy way to write that multiplication is 6! (Yes, an exclamation mark is a math symbol too, it is called factorial)
