[tex]\begin{cases} -x+2y=-7\ \ |\ subtract\ 2y\ to\ both\ sides \\ -2x-6y=-14 \end{cases}\\\\\begin{cases} -x =-2y-7\ \ | \ multiply\ both\ sides\ by\ (-1) \\ -2x-6y=-14 \end{cases}\\\\\begin{cases} x = 2y +7 \\ -2x-6y=-14 \end{cases} \\ \\substitution \\ \\ -2 \cdot(2y +7 ) -6y=-14 \\ -y -14 -6y=-14 \\-7y-14=-14\ \ | \ add\ 14\ to\ both\ sides \\ -7y=0\ \ | \ divide \ both \ sides\ by\ (-7) \\y=0\\\\ x = 2 \cdot 0 +7 \\x=7 \\\\Answer : \ \begin{cases}x=7 \\ y=0 \end{cases} [/tex]
