Question 484848
The log is the exponent.
If you recognize that 81 = 9^2, then 
{{{log(9,81) = 2}}}
----------
If you don't recognize that, or if it's some other number, eg {{{log(9,80)}}},
use the base change formula
--------------
{{{log(9,81) = log(81)/log(9)}}}