25^x-3=125^4-x

I'm assuming we have

25^x-3=125^4-x     Equating bases, we have

(5²)^x-3 = (5³)^4-x    and, by  a property of exponents, we have

5^2x-6 = 5^12-3x       and, equating exponents, we have

2x - 6 = 12 - 3x     add 6, 3x to both sides

5x = 18      divide both sides by 5

x = 18 / 5