In general the way to "move" a number out of an exponent is to use the inverse of exponential: logarithm. If rn= 34, then n log(r)= log(34) so n= log(34)/log(r). It doesn't matter what logarithme you use: common (base 10) or natural (base e) are on your calculator but any base will do.
logs: they're better than bad; they're good! :tongue2: