To simplify \( 6^{-3} \) using the Negative Integer Rule of Exponents, we apply the rule that states \( a^{-n} = \frac{1}{a^n} \).
Thus, we can rewrite:
\[ 6^{-3} = \frac{1}{6^3} \]
So, the simplified form of \( 6^{-3} \) in exponential form is:
\[ \frac{1}{6^3} \]
However, since the task specifies to leave the answer in exponential form, we can express it as:
\[ 6^{-3} \]
Therefore, the answer is:
\[ 6^{-3} \]