Evaluate the following limit. Enter -I if your answer is $-\infty$, enter I if your answer is $\infty$, and enter DNE if the limit does not exist.

$\displaystyle \lim_{ x \rightarrow 0 } \left( \frac{1}{3 x} - \frac{1}{e^{3 x} - 1} \right) =$