Answer
$x=\dfrac{e^6}{3}$
Work Step by Step
Recall:
$$\ln{a}=y \longleftrightarrow e^y=a$$
Use the definition above to obtain:
\begin{align*}
\ln{3x}&=6\\\\
e^6&=3x\\\\
\frac{e^6}{3}&=\frac{3x}{3}\\\\
\frac{e^6}{3}&=x
\end{align*}