#### Answer

$y=\pm9$

#### Work Step by Step

Since $x^2=a$ implies $x=\pm\sqrt{a}$ (the Square Root Property), then the solutions of the given equation, $
y^2=81
,$ are
\begin{array}{l}\require{cancel}
y=\pm\sqrt{81}
\\\\
y=\pm\sqrt{(9)^2}
\\\\
y=\pm9
.\end{array}