Answer
An explicit rule for the sequence is
$ a_n=13(-3)^{n-1}$
Work Step by Step
The given recursive rule is
$a_1=13,a_n=-3a_{n-1}$
First term is $a_1=13$.
and common ratio $r=-3$.
Explicit rule for an geometric sequence.
$\Rightarrow a_n=a_1r^{n-1}$
Substitute $13$ for $a_1$ and $-3$ for $d$.
$\Rightarrow a_n=13(-3)^{n-1}$
