Решить систему: 3x-y=7, x+2y=0

$\left \{ {{3x-y=7} \atop {x+2y=0}} \right.$

$\left \{ {{x+2y=0} \atop {3x-y=7}} \right.$

$\left \{ {{x=-2y} \atop {3\cdot(-2y)-y=7}} \right.$

$3\cdot(-2y)-y=7$

$-6y-y=7$

$-7y=7$

$y=7:(-7)$

$y=-1$

$x=-2y=-2\cdot(-1)=2$

$\left \{ {{x=2} \atop {y=-1}} \right.$

Ответ: (2; -1)

Проверка:

$\left \{ {{3\cdot2-(-1)=7} \atop {2+2\cdot(-1)=0}} \right.$

$\left \{ {{6+1=7} \atop {2-2=0}} \right.$

$\left \{ {{7=7} \atop {0=0}} \right.$