#### Answer

$\text{Set Builder Notation: }
\{ x|x\le12 \}
\\\text{Interval Notation: }
(\infty, 12]$

#### Work Step by Step

Using the properties of inequality, then
\begin{array}{l}\require{cancel}
x+3\le15
\\\\
x\le15-3
\\\\
x\le12
.\end{array}
Hence, the solution set is
\begin{array}{l}\require{cancel}
\text{Set Builder Notation: }
\{ x|x\le12 \}
\\\text{Interval Notation: }
(\infty, 12]
.\end{array}
In the graph, a hollowed dot is used for $\lt$ or $\gt.$ A solid dot is used for $\le$ or $\ge.$