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