Answer
$S_{\infty}=24$
Work Step by Step
Using $S_{\infty}=\dfrac{a_1}{1-r}$, the sum of the infinite geometric sequence $
12,6,3,...
$ is
\begin{array}{l}
S_{\infty}=\dfrac{12}{1-\dfrac{1}{2}}\\\\
S_{\infty}=\dfrac{12}{\dfrac{1}{2}}\\\\
S_{\infty}=24
.\end{array}