Answer:
[tex]\huge\boxed{a=4}[/tex]
Step-by-step explanation:
[tex]\log_xy=z\iff x^z=y\\\\\text{for}\ x\neq1\ \wedge\ x>0\ \wedge\ y>0\\==================\\\\\log_a81=4\iff a^4=81\\\\a^4=\underbrace{3\cdot3\cdot3\cdot3}_{=81}\\\\a^4=3^4\Rightarrow a=3[/tex]
