Chapter 11 - Further Topics in Algebra - 11.3 Geometric Sequences and Series - 11.3 Exercises - Page 1034: 21

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

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