Factor the expression:
x^2 + 4x + 21

Here's your answer!
x2 + 4x - 21

Okay, here's what you have to do.
It's a two termed expression because it's x squared.
So, start your problem off like this.
(x ) (x )

The middle, 4x, and the 21 go together, in a way.
You have to find two numbers that multiplied make 21,
but at the same time can subract to make 4. 
(The only reason it's subtract to make 4 in this problem
is because of the minus sign in front of the 21. That sign
will tell you whether the two numbers should add or 
subract to give you the middle number.)

You're numbers in this case are 7 and 3.
7 times 3 equals 21.
7 minus 3 equals 4. 

I don't exactly remember all the rules with the signs,
but I believe it should look like this:
(x-7) (x-3)

I wouldn't swear on it because it's been awhile, 
but I think that's right.


Hope it helped!