The initial sets are:
[tex]\begin{gathered} X=\mleft\lbrace5,10,15\mright\rbrace \\ Y=\mleft\lbrace3,6,9,12,15\mright\rbrace \end{gathered}[/tex]So the union between them we combinate the items:
[tex]\text{XUY}=\mleft\lbrace3,5,6,9,10,12,15\mright\rbrace[/tex]