#### Answer

$\dfrac {99}{8}$

#### Work Step by Step

$r=\dfrac {\dfrac {-9}{4}}{\dfrac {9}{2}}=\dfrac {\dfrac {9}{2}}{-9}=\dfrac {-9}{18}=-\dfrac {1}{2};a_{1}=18\Rightarrow S_{n}=\dfrac {a_{1}\left( r^{n}-1\right) }{r-1}=\dfrac {18\times \left( \left( \dfrac {-1}{2}\right) ^{5}-1\right) }{-\dfrac {1}{2}-1}=\dfrac {18\times \left( \dfrac {-33}{32}\right) }{-\dfrac {3}{2}}=\dfrac {99}{8}$