Question

...

Answer 1

Since books from the same author have to be put together, let's start by finding how many ways these 4 groups can be arranges:
(possibilities for group 1)×(possibilities for 2)×(possibilities for 3)×(possibilities for 4)

Do you know how to find that?

Answer 2

no...:$

Answer 3

There are 4 groups of books. So, for the first group, there are 4 possibilities, then the second 1 has the 3 that remain, the next has the 2 remaining options, and the last group only has one possible author.
4×3×2×1 = 24 ways to arrange the groups

Answer 4

oh so each group is 4! ?

Answer 5

Since each group has 4 books in them, there are also 24 possible ways to arrange each of the books within the groups. So that leaves us with:
24 (ways to arrange books within groups) × 4 (number of groups) × 24 (ways to arrange the groups) = 2304 ways to arrange the books.

Answer 6

So you can rearrange the order of the books within a group (a group is the books by one author), and you can rearrange the order of the groups.