Step 1 :
After factoring out $ 3 $ we have:
$$ 12x^{2}-75 = 3 ( 4x^{2}-25 ) $$Step 2 :
Rewrite $ 4x^{2}-25 $ as:
$$ 4x^{2}-25 = (2x)^2 - (5)^2 $$Now we can apply the difference of squares formula.
$$ I^2 - II^2 = (I - II)(I + II) $$After putting $ I = 2x $ and $ II = 5 $ , we have:
$$ 4x^{2}-25 = (2x)^2 - (5)^2 = ( 2x-5 ) ( 2x+5 ) $$