Step 1 :
After factoring out $ 9 $ we have:
$$ 36x^{2}-9 = 9 ( 4x^{2}-1 ) $$Step 2 :
Rewrite $ 4x^{2}-1 $ as:
$$ 4x^{2}-1 = (2x)^2 - (1)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 2x $ and $ II = 1 $ , we have:
$$ 4x^{2}-1 = (2x)^2 - (1)^2 = ( 2x-1 ) ( 2x+1 ) $$