5^3x : 25^x+1 = 1/125

5^3x : 25^x+1 = 1/125
=> 5^3x : 5^2(x + 1) = 1/5^3
=> 5^3x : 5^2x+2 = 5^(-3)
=> 5^(3x – (2x + 2)) = 5^(-3)
=> 5^(x – 2) = 5^(-3)
=> x – 2 = -3
=> x = -1