Question

resposta do sistema     x+y=3        x-y=9     

Answer 1

X-y=9
X=y+9

X+y=3
(Y+9)+y=3
2y+9=3
2y=3-9
2y=-6
Y=-6/2
Y=3

X=y+9
X=(3)+9
X=12

Answer 2

x+y=3 ---> x=3-y
x-y=9 ↓

3-y-y=9 x=3-3
-2y=9-3 x=0
-2y=6
y=6/-2

y=-3