Starting equation: 253x+6 = 12512 - 2x

Re-write 25 = 52, and re-write 125 = 53

The above two revisions result in this equation:
[52]3x+6 = [53]12 - 2x
and using Rules for exponents?
52(3x+6) = 53(12 - 2x)

If the powers are equal, and the bases (here, 2) are equal then the exponents must also be equal:

2(3x + 6) = 3(12 - 2x)
6x + 12 = 36 - 6x
12x = 24
x = 2