In order to find this we first can note that the term in the front and the term in the back are both perfect squares. Since the middle term is cancelled out, we can use the difference of two squares to factor.
a^2 - b^2 = (a - b)(a + b)
Since the square root of 9x^2 is 3x, we can use that for a. Since the square root of 16 is 4, we can use that b.
