To use the Power Rule of Exponents, recall that when you raise a power to a power, you multiply the exponents.
The expression given is \( (15^6)^2 \). According to the Power Rule:
\[ (15^6)^2 = 15^{6 \cdot 2} = 15^{12} \]
Therefore, the expression equivalent to \( (15^6)^2 \) is \( 15^{12} \).
Thus, the answer is:
15 superscript 12 baseline.