# How do you solve #x^2 - 21x - 72 = 0#?

##### 2 Answers

Mar 22, 2016

this quadratic equation can be factored as

now that we have a product of two factors equal to zero, that means that at least one of the factors MUST be equal to zero.

so

we have

this means that we have now reduced this to a matter of solving 2 linear equations.

and

Mar 22, 2016

-3 and 24

#### Explanation:

Use the new Transforming Method (Google, Yahoo Search).

The 2 real roots have opposite signs because ac < 0.

Compose factor pairs of (-72) --> (-2, 36)(-3, 24). This last sum is

(24 - 3) = 21 = -b. Then, the 2 real roots are: -3 and 24.

**NOTE**

There is no need to factor by grouping and solving the 2 binomials.