# How do you solve #3x^2-18x-21=0#?

##### 2 Answers

Apr 20, 2016

#### Explanation:

Factor out the GCF

Factor

Find two numbers that when added equal

The numbers

Rewrite the equation.

Solve for

Apr 20, 2016

-1 and 7

#### Explanation:

f(x) = 3y = 3(x^2 - 6x - 7) = 0

Solve the quadratic equation y = 0, in parentheses.

Since a - b + c = 0, use shortcut; the 2 real roots are: -1 and -c/a = 7.

**Reminder of Shortcut**

- When a + b + c = 0 --> 2 real roots: 1 and c/a

- When a - b + c = 0 --> 2 real roots: -1 and -c/a