The Negative Exponent Rule states that \( a^{-n} = \frac{1}{a^n} \), where \( a \) is a non-zero number and \( n \) is a positive integer.
To apply this rule to the expression \( 3^{-12} \):
\[ 3^{-12} = \frac{1}{3^{12}} \]
So, the equivalent expression for \( 3^{-12} \) is:
\[ \frac{1}{3^{12}} \]