How do you factor $15a+21$ ?

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Answer 1 · 2016-05-02T12:16:44

$15a+21= color(blue)(3(5a+7))$

The constants in the terms of the given expression have a common factor of $color(blue)(3)$

Once this factor has been extracted the remaining factor $color(blue)(""(5a+7))$
has no further sub-factors.

How do you factor 15a+2115a+21?