Rewrite $ -9x^{2}+16 $ as:
$$ -9x^{2}+16 = 16 -9x^2 = (4)^2 - (3x)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 4 $ and $ II = 3x $ , we have:
$$ -9x^{2}+16 = (4)^2 - (3x)^2 = ( -3x+4 ) ( 3x+4 ) $$