[tex]\begin{cases} -x+3y=9\\\\ y=\cfrac{2}{3}x \end{cases} \\\\[-0.35em] ~\dotfill\\\\ -x+3y=9\implies 3y=9+x\implies \underline{3y-9=x} \\\\[-0.35em] ~\dotfill\\\\ \stackrel{\textit{substituting ont the 2nd equation}}{y=\cfrac{2}{3}(\underline{3y-9})\implies 3y=2(3y-9)}\implies 3y=6y-18\implies 18+3y=6y \\\\\\ 18=3y\implies \cfrac{18}{3}=y\implies \boxed{6=y}[/tex]
