Rewrite $ -25x^{2}+9 $ as:
$$ -25x^{2}+9 = 9 -25x^2 = (3)^2 - (5x)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 3 $ and $ II = 5x $ , we have:
$$ -25x^{2}+9 = (3)^2 - (5x)^2 = ( -5x+3 ) ( 5x+3 ) $$