I guess the original expression is 125 = 5^x
If you apply logarithms to both sides, you get:
log (125) = x * log (5) <----------- logaritmic equation
=> x = log (125) / log(5)
Given that 125 is 5^3
you can simplify to
x = log (5^3) / log(5) = 3 log (5) / log (5) = 3 * 1 = 3
There you have the expression as a logarithmic equation and the solution of the equation.