Answer
$a=3$
Work Step by Step
Substitute 81 for x and 4 for y.
$4=\log_a{81}$
Rewrite in exponential form.
$a^4=81$
Express 81 as $3^4$.
$a^4=3^4$
Since the powers are the same we can equate the bases.
$a=3$.
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