Solution:
Using the formula,
$\begin{array}{l}
\mathrm{T}_{n}=\mathrm{ar}^{\mathrm{n}-1} \\
\mathrm{a}=18 \\
\mathrm{r}=\mathrm{t}_{2} / \mathrm{t}_{1}=(-12 / 18) \\
=-2 / 3 \\
\mathrm{~T}_{\mathrm{n}}=512 / 729 \\
\mathrm{n}=? \\
\mathrm{~T}_{n}=\mathrm{ar}^{\mathrm{n}-1} \\
512 / 729=18(-2 / 3)^{\mathrm{n}-1} \\
2^{9} /(729 \times 18)=(-2 / 3)^{\mathrm{n}-1} \\
2^{9} / 36 \times 1 / 2 \times 3^{2}=(-2 / 3)^{\mathrm{n}-1} \\
(2 / 3)^{8}=(-1)^{\mathrm{n}-1}(2 / 3)^{\mathrm{n}-1} \\
8=\mathrm{n}-1 \\
\mathrm{n}=8+1 \\
=9
\end{array}$
As a result, $9^{\text {th }}$ term of the Progression is $512 / 729$