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