To simplify the expression \((78^8)^6\) using the Power Rule of Exponents, you apply the rule that states \((a^m)^n = a^{m \cdot n}\).
In this case, you have:
\[ (78^8)^6 = 78^{8 \cdot 6} \]
Now, calculate \(8 \cdot 6\):
\[ 8 \cdot 6 = 48 \]
Thus,
\[ (78^8)^6 = 78^{48} \]
So the simplified expression is:
\[ 78^{48} \]