Answer
See below.
Work Step by Step
If we want to choose $k$ elements out of $n$ disregarding the order, not allowing repetition, we can do this in $_{n}C_k=\frac{n!}{(n-k)!k!}$ ways.
So if we have $7$ CDs and we want to keep $3$ of these we can do this is $_7C_3$ ways.
