Manuel has $54 to buy CDs and books. Each CD costs $9, and each book...

Let c be the number of CDs and b be the number of books that he buys.
Since he buys exactly 7 items, then $b+c=7$.
If each CD costs $9 dollars, then he will spend 9c dollars buying cc CDs. If each book costs $6, then he will spend 6b dollars buying bb books. The total amount he spends is then $6b+9c$. Since he has $54 to spend, then $6b+9c=54$.
The system of equations is then $\{b+c=7,6b+9c=54\}$