Step 1 :
After factoring out $ 2x $ we have:
$$ 18x^{9}-2x = 2x ( 9x^{8}-1 ) $$Step 2 :
Rewrite $ 9x^{8}-1 $ as:
$$ 9x^{8}-1 = (3x^{4})^2 - (1)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 3x^{4} $ and $ II = 1 $ , we have:
$$ 9x^{8}-1 = (3x^{4})^2 - (1)^2 = ( 3x^{4}-1 ) ( 3x^{4}+1 ) $$