Answer: 720 ways
Explanation:
We know that the total number of ways to arrange n things in a row in order is given by Permutations.
i.e. The total number of ways to arrange n things in a row= n!
Therefore, the number of ways to arrange 6 unique CD's in order along a shelf will be :

Hence, there are 720 ways to arrange 6 unique CD's in order along a shelf.