Question

An arithmetic sequence is defined by the recursive formuala t1=9,tn=tn-1-4 where n enand n>1 the seuence s

Answer 1

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

$t_n=9-4(n-1)=13-4n$