#### Answer

$\color{blue}{\text{not a real number}}$

#### Work Step by Step

RECALL:
$\sqrt[n]{a} = a^{1/n}$
Use the rule above by to obtain:
$\sqrt[4]{-81} = (-81)^{1/4}$
Since there is no real number $a$ such that $a^{4} = -81$, then the given expression is:
$\color{blue}{\text{not a real number}}$