Answer
$a_n=-2\cdot 4^{n-1}$ and $a_{5}=-2\cdot 4^{5-1}=-512.$
Work Step by Step
We know that if $a_1$ is the first term and $r$ is the common ratio, then the nth term is: $a_n=a_1\cdot r^{n-1}.$
Hence here $a_n=-2\cdot 4^{n-1}$ and $a_{5}=-2\cdot 4^{5-1}=-512.$