Answer
$2$
Work Step by Step
Note that $81=3^4$. Thus, the given expression is equivalent to:
$=\log_2{(\log_3{(3^4)})}$
RECALL:
$\log_b{b^x} = x$
Use the rule above to obtain:
$=\log_2{4}$
Write $4$ as $2^2$ to obtain:
$=\log_2{(2^2)}$
Use the rule in the RECALL part above to obtain:
$=2$
