Question

How do you factor the expression `x^2+4x-21`?

Answer 1

`(x+7)(x-3)`

Explanation:

To factorise any quadratic polynomial , you could use 2 methods.
Here's the easy one
Let the quadratic polynomial be `ax^2+bx+c`

▪multiply `a" and "c` , then factorise the result into some factors, say y and z, `y+z=|b|`
`x^2+4x-21` `(a×c=-21=-3×7)`

`x^2+7x-3x-21`

`x (x+7)-3 (x+7)`

`(x+7)(x-3)`

The other method is a direct application of quadratic formula.

Answer 2

the answer is `(x+7)(x-3)`

Explanation:

there's an easier way to find this but only when it's `x^2`. since you know that two numbers have to multiply to give you `-21` and add to give you `4` when you find it, you don't have to go through the whole process and you can just put them in the parenthesis since there's no number in front of `x`