Step 1 :
After factoring out $ 6x $ we have:
$$ 24x^{3}-54x = 6x ( 4x^{2}-9 ) $$Step 2 :
Rewrite $ 4x^{2}-9 $ as:
$$ 4x^{2}-9 = (2x)^2 - (3)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 2x $ and $ II = 3 $ , we have:
$$ 4x^{2}-9 = (2x)^2 - (3)^2 = ( 2x-3 ) ( 2x+3 ) $$