Answer
$a_n=7\cdot(0.5)^{n-1}$
Work Step by Step
The nth term of a geometric series can be obtained by the following formula: $a_n=a_1\cdot r^{n-1}$, where $a_1$ is the first term and $r$ is the common ratio.
Here the common ratio is $5$ and $a_1=3$.
Hence here: $a_5/a_3=r^2$, thus $r=0.5$ and $a-3=a_1r^2$ thus $a_1=7$, hence $a_n=7\cdot(0.5)^{n-1}$
