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