Respuesta :

Answer:

[(x + 3) (x + 7)] / (3x + 21)

= [(x + 3) (x + 7)] / [3 (x + 7)]

= (x + 3) / 3

The denominator can't be equal to 0 therefore:

3 (x + 7) ≠ 0 ⇔ x + 7 ≠ 0 ⇔ x ≠ -7