# How do you solve #x^2 + 4x - 21 = 0#?

##### 3 Answers

We are going to solve it with

The equation is of the form

You can solve this by using the general formula for complete quadratic equations.

For a complete quadratic equation

Let's apply it to your equation, which gives:

So, we've got two solutions:

and

A less direct, but perhaps simpler approach is to recognise that if

The reason I say it must look like this with one

Then

What pair of factors of

There are not many choices of factorizations, basically either

This is zero when