[tex]\it b)\ \ \begin{cases} \it3x+4y=6\\ \\ \it x-4y=-14\end{cases}\\ \rule{0.2}{0.2}\ \ \ \ \ \ \rule{80}{0.7}\\ \rule{0.2}{0.2}\ \ \ \ \ \ \ 4x\ \ \ \ \Big|\ \ \ =-8 \Rightarrow x=-2\ \ \ \ (*)\\ \\ x-4y=-14\ \stackrel{(*)}{\Longrightarrow}\ -2-4y=-14 \Rightarrow -2+14=4y \Rightarrow 12=4y \Rightarrow y=3\\ \\ S=\{(-2,\ 3)\}[/tex]
