To perform the operation \( 3\sqrt{5} \times 7\sqrt{11} \), we can follow these steps:
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Multiply the coefficients (the numbers outside the square roots): \[ 3 \times 7 = 21. \]
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Multiply the square roots together: \[ \sqrt{5} \times \sqrt{11} = \sqrt{5 \times 11} = \sqrt{55}. \]
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Combine the results: \[ 3\sqrt{5} \times 7\sqrt{11} = 21\sqrt{55}. \]
Since \( \sqrt{55} \) is already in its simplest form and there is no need to rationalize the denominator here (as there is no denominator), the final answer is: \[ \boxed{21\sqrt{55}}. \]