To simplify the expression \(\sqrt{81y^6}\), we can break it down into two parts: the square root of the numerical part and the square root of the variable part.
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Start with the numerical part: \[ \sqrt{81} = 9 \]
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Next, simplify the variable part: \[ \sqrt{y^6} = y^{6/2} = y^3 \]
Now, we can combine both parts: \[ \sqrt{81y^6} = \sqrt{81} \cdot \sqrt{y^6} = 9y^3 \]
Therefore, the simplified expression is: \[ \boxed{9y^3} \]