Question

Solve for x.

x2 + 4x - 21 = 0

A. 

-3, -7

B. 

-3, 7

C. 

3, -7

D. 

3, 7

​

Answer 1

C. 3, -7

Explanation:

`x^2+4x-21=0\\\\x^2+7x-3x-21=0\\\\x(x+7)-3(x+7)=0\\\\(x+7)(x-3)=0\iff x+7=0\ \vee\ x-3=0\\\\x+7=0\qquad\text{subtract 7 from both sides}\\x=-7\\\\x-3=0\qquad\text{add 3 to both sides}\\x=3`

Answer 2

C. 3, -7

Explanation:

x^2 +4x-21 =0

Factor

What 2 numbers multiply to -21 and add to 4

7*-3 = -21

7-3 = 4

(x+7) (x-3) =0

Using the zero product property

x+7 =0 x-3=0

x+7-7=0-7 x-3+3=0+3

x = -7 x=3