# How do you solve #x^2 - 4x - 21<=0# A) [-3, 7] B) (-∞, -3] C) (-∞, -3] [7, ∞) D) [7, ∞)?

##### 1 Answer

Aug 16, 2015

Solve

Ans: [-3, 7]

#### Explanation:

First, solve

Factor pairs of (-21) --> (-3, 7). This sum is 4 = -b. Then, the 2 real roots are: -3 and 7.

Between the 2 real roots (-3) and (7), f(x) has the opposite sign of a = 1. Then

The 2 critical points (-3) and (7) are included in the solution set.