# Factorizing

• January 29th 2009, 01:36 AM
positiveion
Factorizing
I'm not sure how to do factorize this problem:
X^2 + 4X - 21

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.