Question

find the 5th term of the sequence in which t1 = 8 and tn-1 = t= -3tn-1

Answer 1

I think you mean to say that the sequence is given recursively by

$\begin{cases}t_1=8\\t_n=-3t_{n-1}\end{cases}$

From this you can find a general pattern for the $n$th term, but since we're only interested in determining $t_5$, we can stop once we arrive at that. We have

$t_2=-3t_1$
$t_3=-3t_2=(-3)^2t_1$
$t_4=-3t_3=(-3)^3t_1$
$t_5=-3t_4=(-3)^4t_1$

$\implies t_5=8(-3)^4=8(81)=648$