#### Answer

$a_{5}=a_{1}\times r^{4}=-9\times \left( -3\right) ^{4}=-729;$
$a_{n}=a_{1}\times r^{n-1}=\left( -9\right) \times \left( -3\right) ^{n-1}$

#### Work Step by Step

$a_{1}=\dfrac {a_{4}}{r^{3}}=\dfrac {243}{\left( -3\right) ^{3}}=-9;a_{5}=a_{1}\times r^{4}=-9\times \left( -3\right) ^{4}=-729;a_{n}=a_{1}\times r^{n-1}=\left( -9\right) \times \left( -3\right) ^{n-1}$