Find the common ratio.

$\frac{6}{2}=3$

$\frac{18}{6}=3$

Using $t(n)=a·r^{n−1}$, find $t\left(6\right)$.

Now find the sum of the geometric series using:

$S(k)=\frac{r\cdot t(k)-t(1)}{r-1}$