Rewrite $ -25n^{2}+1 $ as:
$$ -25n^{2}+1 = 1 -25x^2 = (1)^2 - (5n)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 1 $ and $ II = 5n $ , we have:
$$ -25n^{2}+1 = (1)^2 - (5n)^2 = ( -5n+1 ) ( 5n+1 ) $$