Answer
$15$
Work Step by Step
$\bf{\text{Solution Outline:}}$
To evaluate the given expression, $
x+y
,$ substitute the given values $x=3$ and $y=12.$
$\bf{\text{Solution Details:}}$
Substituting $x=3$ and $y=12,$ the given expression evaluates to
\begin{array}{l}\require{cancel}
x+y
\\\\=
3+12
\\\\=
15
.\end{array}
