Question

qual o Valor de x + 2y = 30
x+ y = 12 ???

Answer 1

x+ 2 y = 30
x+ y = 12 (-1)

x+ 2 y = 30
- x- y = -12 
    y = 18
    
x+ y = 12 ==> x = 12 - y ==> x = 12 - 18 ==> x = - 6

     

Answer 2

$\left \{ {{x+2y=30} \atop {x+y=12}} \right.$

$\left \{ {{x+2y=30} \atop {-x-y=-12}} \right. (-1)$

y= 30-12
y=18

x+y= 12
x+18= 12
x= -18+12
x= -6