Answer
$x=4$
Work Step by Step
\begin{align*}
\ln{e^{3x}}&=12 &\text{Take the natural logarithm of both sides.}\\\\
3x\ln{e}&=12 &\text{Use the Power Property of Logarithms.}\\\\
3x(1)&=12 &\text{Use the rule $\ln{e}=1$.}\\\\
3x&=12\\\\
\frac{3x}{3}&=\frac{12}{3} &\text{Divide 3 to both sides.}\\\\
x&=4
\end{align*}
