Question

$\left \{ {{x+7y=-2} \atop {-x-4y=-1}} \right.$

Answer 1

Sistemas 
por adição

corta o x e o -x

3y = - 3
y = -3/3
y = - 1

x  + 7y = - 2
x + 7 (-1) = -2
x - 7 = - 2
x = - 2 + 7
x = 5

Answer 2

Somando os dois sistemas :

$\left \{ {{x+7y=-2} \atop {-x-4y=-1}} \right. \\\\3y=-3\\\\\boxed{y=-1}\\\\x-7=-2\\x=-2+7\\\\\boxed{x=5}\\\\5-7=-2\\-2=-2$