#### Answer

$4\sqrt[]{y+3}$

#### Work Step by Step

Simplifying each term and then combining like terms, the given expression, $
\sqrt[]{9y+27}+\sqrt[]{y+3}
,$ is equivalent to
\begin{array}{l}\require{cancel}
\sqrt[]{9(y+3)}+\sqrt[]{y+3}
\\\\=
\sqrt[]{(3)^2\cdot(y+3)}+\sqrt[]{y+3}
\\\\=
3\sqrt[]{y+3}+\sqrt[]{y+3}
\\\\=
4\sqrt[]{y+3}
.\end{array}
* Note that it is assumed that all variables represent positive numbers.