[tex]\it b)\ \ \begin{cases} \it 5x-2y=-5\ \ \ \ \ (1)\\ \\ \it -10x+3y=8\ \ \ \ \ \ \ (2)\end{cases}\\ \\ \\ (1),\ (2)\ \stackrel{(+)}{\Longrightarrow}\ -5x+y=3 \Rightarrow y=5x+3\ \ \ \ \ (3)\\ \\ \\ (2),\ (3) \Rightarrow -10x+3(5x+3)=8 \Rightarrow -10x+15x+9=8\bigg|_{-9} \Rightarrow\\ \\ \\ \Rightarrow 5x=-1 \Rightarrow x=-\dfrac{1}{5}\ \ \ \ \ (4)[/tex]
[tex]\it (3),\ (4) \Rightarrow y=5\cdot\bigg(-\dfrac{1}{5}\bigg)+3=-1+3=2\\ \\ \\ S=\bigg\{(-\dfrac{1}{5},\ 2)\bigg\}[/tex]
[tex]\it c)\ \ \begin{cases} \it 7x-9y=23\ \ \ \ \ (1)\\ \\ \it 3x+5y=1\ \ \ \ \ \ \ (2)\end{cases}\\ \\ \\ (1),\ (2)\ \stackrel{(+)}{\Longrightarrow}\ 10x-4y=24\bigg|_{:4} \Rightarrow 2,5x-y=6 \Rightarrow y=2,5x-6\ \ \ \ \ (3)\\ \\ \\ (2),\ (3) \Rightarrow 3x+5(2,5x-6)=1 \Rightarrow 3x+12,5x-30=1 \Rightarrow[/tex]
[tex]\it \Rightarrow 15,5x=31\bigg|_{\cdot2} \Rightarrow 31x=62\bigg|_{:31} \Rightarrow x=2\\ \\ x=2\ \stackrel{(3)}{\Longrightarrow}\ y=2,5\cdot2-6 \Rightarrow y=5-6 \Rightarrow y=-1\\ \\ \\ S=\{(2,\ \ -1)\}[/tex]
