To multiply the expressions \( 9\sqrt{3} \) and \( 6\sqrt{7} \), you can follow these steps:
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Multiply the coefficients (the numbers outside the square roots): \[ 9 \cdot 6 = 54 \]
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Multiply the square roots: \[ \sqrt{3} \cdot \sqrt{7} = \sqrt{3 \cdot 7} = \sqrt{21} \]
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Combine the results: \[ 9\sqrt{3} \cdot 6\sqrt{7} = 54\sqrt{21} \]
Thus, the exact answer is: \[ \boxed{54\sqrt{21}} \]