Question

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

Answer 1

Note that `9x^2 = (3x)^2`
and
`121 = 11^2`

So
`9x^2 -121` is the difference of two squares.

In its general form the difference of two squares:
`(a^2-b^2) = (a+b)(a-b)`

So
`9x^2-121`
`= (3x)^2-11^2`
`= (3x+11)(3x-11)`