To rewrite \((6^9)^2\) using a single exponent, you can apply the power of a power rule, which states that \((a^m)^n = a^{m \cdot n}\).
In this case:
\[ (6^9)^2 = 6^{9 \cdot 2} = 6^{18} \]
So, the expression \((6^9)^2\) can be rewritten as \(6^{18}\).