It really depends on the question and what you want to solve. An exponential function can be written as y = x^2 and if you are solving for y then it will be log(base x) y = 2.
More Examples: 3x = 15 (log both sides with base of 3) log(3)3x = log(3)15 since log(3)3 = 1 then, (1)x = log(3)15 x = log 15 / log 3 = 2.465
