# How do you factor 3a^3b + 21a^2b^4?

$3 {a}^{2} b \left(a + 7 {b}^{3}\right)$
$3 {a}^{3} b + 21 {a}^{2} {b}^{4}$
$= 3 \left({a}^{3} b + 7 {a}^{2} {b}^{4}\right)$
$= 3 {a}^{2} \left(a b + 7 {b}^{4}\right)$
$= 3 {a}^{2} b \left(a + 7 {b}^{3}\right)$