Answer
$\frac{1456}{81}$.
Work Step by Step
The sum of a geometric sequence until $n$ can be obtained by the formula $S_n=a_1(\frac{1-r^n}{1-r})$ where $a_1$ is the first term and $r$ is the common ratio. Hence here the sum is: $S_n=12(\frac{1-(\frac{1}{3})^6}{1-\frac{1}{3}})=\frac{1456}{81}$.
