Factorize the expression x2 - 9.

In $x^2 - 9$, we have $a^2 = x^2$, so $a = x$.
We also have $b^2 = 9$, so $b = \sqrt{9} = 3$.

Substitute $a=x$ and $b=3$ into the formula $(a - b)(a + b)$.

Comparing this with the given options:
a. $(x+3)(x-3)$
b. $(x-1)(x-9)$
c. $(x-3)(x-3)$
d. $(x+9)(x-1)$

Option a is equivalent to our result, as the order of multiplication does not change the product.

The final answer is $\boxed{\text{a. } (x+3)(x-3)}$.