Answer
$6a^2\sqrt{b}$
Work Step by Step
Using the properties of radicals, the given expression, $
\sqrt{36a^4b}
,$ simplifies to
\begin{array}{l}\require{cancel}
\sqrt{36a^4\cdot b}
\\\\=
\sqrt{(6a^2)^2\cdot b}
\\\\=
6a^2\sqrt{b}
\end{array}
* Note that it is assumed that no radicands were formed by raising negative numbers to even powers.