The expression \(9x^2 - 1\) is a quadratic expression that can be factored using the difference of squares formula. The difference of squares formula states that \(a^2 - b^2 = (a - b)(a + b)\).

In this case, we can rewrite \(9x^2 - 1\) as \((3x)^2 - 1^2\). Here, \(a = 3x\) and \(b = 1\).

Using the difference of squares formula, we get:

\[9x^2 - 1 = (3x)^2 - 1^2 = (3x - 1)(3x + 1)\]

So, the factored form of \(9x^2 - 1\) is:

\[(3x - 1)(3x + 1)\]