Question

how do you factor 9x^2 - 16

Answer 1

(3x+4)(3x-4)

Answer 2

first rewrite as (3x)^2-4^2

Answer 3

then you have difference of two squares

Answer 4

you should know that a^2-b^2=(a-b)(a+b)

Answer 5

a=3x and b=4 in your case and there you go

Answer 6

you should get to the result that @BackTo90sRegime achieved!
although i'm not a fan of giving the answer nor is it allowed here in OP