#### Answer

$3$

#### Work Step by Step

$\because y=\log_a x \text{ is equivalent to } x= a^y$
$\therefore x = \log_4 64 \text{ is equivalent to } 64=4^x$
Solve the equation above using the rule $a^m=a^n\implies m=n$ to obtain:
\begin{align*}
64&=4^x\\\\
4^3&=4^x\\\\
3&=x\end{align*}