Number of books: B
Number of students: S
We know that the classroom library has 17 more books than 3 times the number of students. Then:
[tex]B=3S+17[/tex]Additionally, if 5 students are absent, each student can borrow exactly 4 books. Then:
[tex]B=4(S-5)=4S-20[/tex]Now, using these two results:
[tex]\begin{gathered} 3S+17=4S-20 \\ 17+20=4S-3S \\ 37=S \end{gathered}[/tex]There are 37 students in the class.