Step 1 :
After factoring out $ x $ we have:
$$ 9x^{3}-25x = x ( 9x^{2}-25 ) $$Step 2 :
Rewrite $ 9x^{2}-25 $ as:
$$ 9x^{2}-25 = (3x)^2 - (5)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 3x $ and $ II = 5 $ , we have:
$$ 9x^{2}-25 = (3x)^2 - (5)^2 = ( 3x-5 ) ( 3x+5 ) $$