Factor completely 3x² - 21.
3(x² - 7), 3(x + 7)(x - 7), 3(x + 7)(x - 3), Prime 

Solution:

Given: Expression is 3x² - 21

In order to find roots, we need to factor the polynomial.

⇒ 3x² - 21

⇒ 3(x² - 7)

This is in the form (a² - b²), we know that the difference of the squares (a² - b²) = (a + b)(a - b)

We can write (x² - 7) as (x + 7) and (x - 7)

⇒ 3(x - 7)(x + 7)

Hence, by factoring completely, we get 3(x - 7)(x + 7).

Factor completely 3x² - 21.

Summary:

Factors of 3x² - 21 are 3(x - 7)(x + 7).