f(x) = 36x^2-324\:
We know that square of 6x is 36x^2 and square of 18 is 324.
\left(6x\right)^2-\:\left(18\right)^2
We know that, a^2-b^2\:=\:\left(a+b\right)\left(a-b\right)
\left(6x+18\right)\:\left(6x-18\right)
6(x+3) 6(x-3)
36(x+3)(x-3)
Factor of the expression f(x) = 36x^2-324\: is
(6x + 18) (6x-18) or
36(x+3)(x-3)