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