Rewrite $ 25x^{2}-9 $ as:
$$ 25x^{2}-9 = (5x)^2 - (3)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 5x $ and $ II = 3 $ , we have:
$$ 25x^{2}-9 = (5x)^2 - (3)^2 = ( 5x-3 ) ( 5x+3 ) $$