EDIT: ** Baah. When I typed this answer I was first. Then at the end, I'm like the 3rd. Oh well..**
There are two ways. Which one is easier depends greatly on your thinking skills.
You can take the expression x2+7x+12 and assume it should break up in two factors:
(x+ ? )(x+ ?)
Then, search for two numbers that add up to the 7, but when multiplied equal 12.
It helps if you break down 12 into it's components, 12= 2*2*3. So, obviously, the answers could be:
2 * (2*3) = 2 * 6 (but 2+6 is not =7)
3 * (2*2) = 3 * 4, voila! 3+4 also = 7, so:
(x+3)(x+4).
The second way could be just solving the cuadratic formula for second degree equations:
b=(-b +/- SQRT(b2 - 4ac))/2a
Where ax2 + bx + c, so you'll have: a=1, b=7, c=12.
Replace, then solve twice: Once for the + value of the SQRT, another for the - value of the SQRT. You should get something like:
First run:
x= -3
Second run:
x= -4
x=-3 is the same thing as (x+3)=0
and
x=-4 is also (x+4)=0
Notice the two factors are precisely (x+3) and (x+4)?
Hope this helped..