To simplify the expression \((15^6)^2\) using the Power Rule of Exponents, which states that \((a^m)^n = a^{m \cdot n}\), we can apply this rule as follows:
\[ (15^6)^2 = 15^{6 \cdot 2} = 15^{12}. \]
Therefore, the equivalent expression is:
15 superscript 12 baseline.