Answer
$4$
Work Step by Step
For $a>0, a\neq 1$, and $x>0,\qquad y=\log_{a}x$ means $a^{y}=x$.
$.................................$
$\log_{3}81=x$ means $3^{x}=81$,
$3^{x}=81$
$3^{x}=3^{4}$
$x=4$
With change of base, $\displaystyle \log_{a}x=\frac{\log_{b}x}{\log_{b}a}=\frac{\ln x}{\ln a}$,
using a calculator,
$\log_{3}81$=$\displaystyle \frac{\ln 81}{\ln 3}=4$
