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?
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.