Step 1 :
After factoring out $ 2x $ we have:
$$ 18x^{3}-32x = 2x ( 9x^{2}-16 ) $$Step 2 :
Rewrite $ 9x^{2}-16 $ as:
$$ 9x^{2}-16 = (3x)^2 - (4)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 3x $ and $ II = 4 $ , we have:
$$ 9x^{2}-16 = (3x)^2 - (4)^2 = ( 3x-4 ) ( 3x+4 ) $$