Answer
4
Work Step by Step
For any positive real number $b$ (such that $b\ne1$), we know that $log_{b}b^{r}=r$ (as long as $r\gt0$).
For $log_{3}81$:
We know that $3^{4}=81$, so we can replace 81 with $3^{4}$.
Therefore, $log_{3}81=log_{3}3^{4}=4$.