Rewrite $ -25d^{4}+1 $ as:
$$ -25d^{4}+1 = 1 -25x^4 = (1)^2 - (5d^{2})^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 1 $ and $ II = 5d^{2} $ , we have:
$$ -25d^{4}+1 = (1)^2 - (5d^{2})^2 = ( -5d^{2}+1 ) ( 5d^{2}+1 ) $$