# How do you factor 9x^2-121?

##### 1 Answer
May 1, 2015

Note that $9 {x}^{2} = {\left(3 x\right)}^{2}$
and
$121 = {11}^{2}$

So
$9 {x}^{2} - 121$ is the difference of two squares.

In its general form the difference of two squares:
$\left({a}^{2} - {b}^{2}\right) = \left(a + b\right) \left(a - b\right)$

So
$9 {x}^{2} - 121$
$= {\left(3 x\right)}^{2} - {11}^{2}$
$= \left(3 x + 11\right) \left(3 x - 11\right)$