How do you factor #x^2 + 4x -21#?

1 Answer
EZ as pi
Jun 24, 2016

#(x+7)(x-3)#

Explanation:

Find factors of 21 which subtract (because of the minus sign)
to give 4. (the middle term)

The signs in the brackets must be different (because of the minus sign), but there must be more positives. (sign of the middle term.)

Factors of 21 are: #1 xx 21, or 7 xx3#
7 and 3 are the ones we want.

#7 xx3 = 21 and 7 - 3 =4#

#(x+7)(x-3)#

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