Factorizing

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• January 29th 2009, 02:36 AM
positiveion
Factorizing
I'm not sure how to do factorize this problem:
X^2 + 4X - 21

The answer is:
(X-3)(X+7)

So I tried to split it up like this:
X(X+1)-3(X+7)

.. but I'm not sure how to group it.

Help please?
• January 29th 2009, 02:51 AM
james_bond
You should find its roots. $3^2+4\cdot 3-21=0\text{ and }(-7)^2-7\cdot 4-21=0$
Do you know the quadratic formula? If not you can still figure the factor out: first $(x\pm?)(x\pm?)$ second: $?\cdot ?$ must be $-21$ so you try with integers and get it soon.