To make the two expressions \( (3^3)x \) and \( 3^{24} \) equivalent, we can rewrite the first expression.
Starting with:
\[ (3^3)x = 3^{24} \]
We can express \( x \) in terms of \( 3^{n} \) as follows:
\[ 3^{3} \cdot x = 3^{24} \]
To isolate \( x \), we can divide both sides by \( 3^{3} \):
\[ x = \frac{3^{24}}{3^{3}} = 3^{24 - 3} = 3^{21} \]
Hence, the exponent that would make the two expressions equivalent is:
\[ \boxed{21} \]