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?

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- Jan 29th 2009, 01:36 AMpositiveionFactorizing
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? - Jan 29th 2009, 01:51 AMjames_bond
You should find its roots. $\displaystyle 3^2+4\cdot 3-21=0\text{ and }(-7)^2-7\cdot 4-21=0$

Do you know the quadratic formula? If not you can still figure the factor out: first $\displaystyle (x\pm?)(x\pm?)$ second: $\displaystyle ?\cdot ?$ must be $\displaystyle -21$ so you try with integers and get it soon.