Answer
A recursive rule for the sequence is
$a_1=7,a_n=3a_{n-1}$
Work Step by Step
The given explicit rule is
$a_n=7(3)^{n-1}$
First term is
$a_1=7$
Common ratio is
$r=3$
Recursive equation for a geometric sequence.
$a_n=r\cdot a_{n-1}$
Substitute $3$ for $r$.
$a_n=3\cdot a_{n-1}$
Hence, a recursive rule for the sequence is
$a_1=7,a_n=3a_{n-1}$
