[tex]\it 3x+y=7
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2x-y=8\\
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5x \ \ \ \ =15 \Rightarrow x = 15:5 \Rightarrow x = 3\ \ \ \ \ (*)[/tex]
[tex]\it 3x+y=7 \stackrel{(*)}{\Longrightarrow} 3\cdot3+ y=7 \Rightarrow 9+y=7 \Rightarrow y = 7 -9 \Rightarrow y = -2
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S: \begin{cases}\it x=3\\ \\ \it y=-2 \end{cases}[/tex]