Which expression is the factorization of x2 + 10x + 21?

To factor x² + 10x + 21, find two numbers whose product is 21 and whose sum is 10. Those numbers are 3 and 7, so x² + 10x + 21 = (x + 3)(x + 7). Therefore, the correct choice is (x + 3)(x + 7).

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