You need to re-write each side of the equation such that the base is 5.
25 = 5² and 1/5 = 5-1
This problem now becomes:
(5²)6 - x² = (5-1)2x
5(12 - 2x²) = 5-2x
If the bases are even, then we can set the exponents equal to each other.
12 - 2x² = -2x
12 + 2x - 2x² = 0
6 + x - x² = 0
(3 - x)(2 + x) = 0
x = 3 or x = -2