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
\(\begin{cases}t_1=9\\t_n=t_{n-1}-4&\text{for }n>1\end{cases}\)
you can do that by solving for \(t_n\) explicitly.
\(t_n=t_{n-1}-4=t_{n-2}-2\times4=t_{n-3}-3\times4=\cdots=t_1-(n-1)\times4\)
So the explicit formula for the \(n\)th term is
\(t_n=9-4(n-1)=13-4n\)
