Resposta :
[tex]Resuminho..\\\\\begin{bmatrix}x+y=12\\ 3x-y=20\end{bmatrix}\\\\\begin{bmatrix}3\left(12-y\right)-y=20\end{bmatrix}\\\\\begin{bmatrix}36-4y=20\end{bmatrix}\\\\\mathrm{Para\:}x=12-y\\\\\mathrm{Substituir\:}y=4\\\\x=12-4\\\\x=8\\\\x=8,\:y=4[/tex]
RESPOSTA > B) S= {8, 4}