Answer
$S_{4}=720$
Work Step by Step
$\displaystyle \sum_{i=1}^{4}6(3)^{i}=6(3)^{1}+6(3)^{2}+6(3)^{3}+6(3)^{4}$
$a_1=6(3)^1=18$
$r=\frac{a_2}{a_1}=\frac{6(3)^2}{6(3)^1}=3$
$S_{4}=a_1(\frac{1-r^n}{1-r})=18(\frac{1-3^4}{1-3})=18\frac{-80}{-2}=720$
