Answer:
3√7
Explanation:
(√3)(√21) = √(3·21) = √(3·3·7) = √(3²·7) = 3√7
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The product of the roots is the root of the product. The product, 3·21 = 63, has a square factor that can be removed from under the radical. You need to factor out the root of that square when you simplify the expression.