It's not clear what you're supposed to do, but I would guess that you're asked to find a certain term in the sequence.
Given
[tex]\begin{cases}t_1=9\\t_n=t_{n-1}-4&\text{for }n>1\end{cases}[/tex]
you can do that by solving for [tex]t_n[/tex] explicitly.
[tex]t_n=t_{n-1}-4=t_{n-2}-2\times4=t_{n-3}-3\times4=\cdots=t_1-(n-1)\times4[/tex]
So the explicit formula for the [tex]n[/tex]th term is
[tex]t_n=9-4(n-1)=13-4n[/tex]
