Logarithm with a variable base

I want to make a logarithm that has a variable as its base, like:

k=2

logk(8)

ans =

3

How can I do that?

Answer 1

[Voss]

k=2
log(8)/log(k)

[Voss]

See the first property listed here: http://dl.uncw.edu/digilib/Mathematics/Algebra/mat111hb/EandL/logprop/logprop.html