Answer:
(7x + 2)(x - 2)(x + 2)
Step-by-step explanation:
Given
7x³ + 2x² - 28x - 8 ( factor the first/second and third/fourth terms )
= x²(7x + 2) - 4(7x + 2) ← factor out (7x + 2) from each term
= (7x + 2)(x² - 4) ← x² - 4 is a difference of squares
= (7x + 2)(x - 2)(x + 2)
