Ответ:

[tex]\left \{ {{x+y=0} \atop {3x-y=4}} \right.[/tex]

4x=4

x= 4 : 4

x=1

[tex]\left \{ {{x=1} \atop {1+y=0}} \right.[/tex]

1+y=0

y= -1

Ответ: ( 1 ; -1 )

Проверим

[tex]\left \{ {{1+(-1)=0} \atop {3*1-(-1)=4}} \right.[/tex]

[tex]\left \{ {{1-1=0} \atop {3*1+1=4}} \right.[/tex]

[tex]\left \{ {{0=0} \atop {4=4}} \right.[/tex]