Question

$log _{x} 81 = 4$

Answer 1

$log_{x}= 81=4 \\ x^4= 81 \\ x^4= 3^4 \\ x=3$

Answer 2

Lembrando: $log_{b}(a)=c <=> b^{c}=a$
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$log_{x}(81)=4$
$x^{4}=81$
$x^{4}=3^{4}$
$x=3$