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