Answer
$S_6=1092$
Work Step by Step
Using $S_n=\dfrac{a_1(1-r^n)}{1-r}$, the sum of the first $
6
$ terms of the sequence $
3,6,9,...
$ is
\begin{array}{l}
S_6=\dfrac{3(1-(3)^6)}{1-3}\\\\
S_6=\dfrac{3(-728)}{-2}\\\\
S_6=1092
.\end{array}