Answer
$S_{100}=-30,000$
Work Step by Step
Using $S_n=\dfrac{n}{2}(2a_1+(n-1)d)$, with $a_1=-3$ and $d=-6$, then,
\begin{array}{l}
S_{100}=\dfrac{100}{2}(2(-3)+(100-1)(-6))\\\\
S_{100}=50(-6+(-594))\\\\\
S_{100}=50(-600)\\\\\
S_{100}=-30,000
.\end{array}