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