Which factoring method would be best to factor x^2 - 9?

A perfect square trinomial method

B greatest common factor method

C trial and error

D difference and squares

D. difference of squares

Step-by-step explanation:

Both of the terms x^2 and 9 are squares, so their difference is the difference of squares. The factorization of that is a "special form" that can be usefully memorized.

a^2 - b^2 = (a - b) (a + b)

x^2 - 9 = (x - 3) (x + 3)