Question

2x+y=15
y=3x
I think it's substitution

Answer 1

`2x+y=15 \\ 2x+3x=15 \\ 5x=15 \\ /5\ \ /5 \\ x=3`

`y=3x \\ x=3 \\ y=3*3=9 \\ y=9`

Answer 2

`\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\`