Answer :

VzDeath
[tex]2x+y=15 \\ 2x+3x=15 \\ 5x=15 \\ /5\ \ /5 \\ x=3[/tex]
[tex]y=3x \\ x=3 \\ y=3*3=9 \\ y=9[/tex]
[tex]\left\{\begin{array}{ccc}2x+y=15\\y=3x\end{array}\right\\\\subtitute:\\\\2x+3x=15\\5x=15\ \ \ \ |divide\ both\ sides\ by\ 5\\\boxed{x=3}\\\\subtitute\ value\ of\ "x"\ to\ the\ equation\ y=3x:\\y=3\cdot3=9\\\\Answer:\boxed{ \left\{\begin{array}{ccc}x=3\\y=9\end{array}\right}[/tex]