#### Answer

$a_{5}=a_{1}\times r^{5-1}=-4\times 3^{4}=-324;$
$a_{n}=a_{1}\times r^{n-1}=-4\times 3^{n-1}$

#### Work Step by Step

$r=-\dfrac {108}{-36}=\dfrac {-36}{-12}=\dfrac {-12}{-4}=3;a_{1}=-4\Rightarrow a_{5}=a_{1}\times r^{5-1}=-4\times 3^{4}=-324;a_{n}=a_{1}\times r^{n-1}=-4\times 3^{n-1}$