We have, x 2 - 4x – 21 = x 2 + 3x - 7x – 21 = x (x + 3) - 7 (x + 3) = (x + 3) (x - 7) Hence, option C is correct

x 2 - 4x – 21 = ?

We have,

x² - 4x – 21

= x² + 3x - 7x – 21

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

= (x + 3) (x - 7)

Hence, option C is correct