$$625^{n+2} - 125^{n+2} - 25^{n+2} = (5^4)^{n+2} - (5^3)^{n+2} - (5^2)^{n+2} = 5^{4n+8} - 5^{3n+6} - 5^{2n+4} = 5^{2n+4}(5^{2n+4} - 5^{n+2} - 1)$$
Ответ: $$5^{2n+4}(5^{2n+4} - 5^{n+2} - 1)$$.