To simplify the expression \((7^3)^9\) using the Power Rule of Exponents, which states that \((a^m)^n = a^{m \cdot n}\), we proceed as follows:
\[ (7^3)^9 = 7^{3 \cdot 9} = 7^{27} \]
So, the simplified expression is:
7 superscript 27 baseline (7^27).