hmmmm let's see \(\bf \textit{sum of a finite geometric sequence}=S_n=a_1\left( \cfrac{1-r^n}{1-r} \right)
\\\\
\begin{cases}
n=n^{th}\ term\\
a_1=\textit{first term}\\
r=\textit{common ratio}
\end{cases}\qquad thus\implies S_8=9\left( \cfrac{1-(-3)^8}{1-(-3)} \right)\)