Step 1 :
After factoring out $ 2x $ we have:
$$ 50x^{5}-18x = 2x ( 25x^{4}-9 ) $$Step 2 :
Rewrite $ 25x^{4}-9 $ as:
$$ 25x^{4}-9 = (5x^{2})^2 - (3)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 5x^{2} $ and $ II = 3 $ , we have:
$$ 25x^{4}-9 = (5x^{2})^2 - (3)^2 = ( 5x^{2}-3 ) ( 5x^{2}+3 ) $$