To simplify the expression \((15^6)^2\) using the Power Rule of Exponents, you multiply the exponents.
According to the Power Rule: \((a^m)^n = a^{m \cdot n}\).
So in this case:
\[ (15^6)^2 = 15^{6 \cdot 2} = 15^{12} \]
Therefore, the equivalent expression is:
15^12