[tex]\it f)\ \ \begin{cases} \it x-2y=-8\ \Rightarrow\ x=2y-8\ \ \ \ \ (*)\ \ rela\c{\it t}ia\ de\ substitu\c{\it t}ie\\ \\ \it 3x+4y=-4\end{cases}\\ \\ \it 3x+4y=-4\ \stackrel{(*)}{\Longrightarrow}\ 3(2y-8)+4y=-4\ \Rightarrow\ 6y-24+4y=-4\ \Rightarrow\\ \\ \ \Rightarrow\ 6y+4y=24-4\ \Rightarrow\ 10y=20|_{:{10}}\ \Rightarrow\ y=2\\ \\ y=2\ \stackrel{(*)}{\Longrightarrow}\ x=2\cdot2-8=4-8=-4\\ \\ S=\{-4,\ 2\}[/tex]
