#### Answer

$\dfrac{3\sqrt{x+y}}{x+y}$

#### Work Step by Step

$\bf{\text{Solution Outline:}}$
To rationalize the denominator of the given expression, $
\dfrac{3}{\sqrt{x+y}}
,$ multiply both the numerator and the denominator by the denominator.
$\bf{\text{Solution Details:}}$
Multiplying both the numerator and the denominator by the denominator, the given expression is equivalent to
\begin{array}{l}\require{cancel}
\dfrac{3}{\sqrt{x+y}}\cdot\dfrac{\sqrt{x+y}}{\sqrt{x+y}}
\\\\=
\dfrac{3\sqrt{x+y}}{(\sqrt{x+y})^2}
\\\\=
\dfrac{3\sqrt{x+y}}{x+y}
.\end{array}