Respuesta :

3x-4y=18
3x-y= 18/4 = 9/2
x-y = 9/2 / 3/1
x-y= 3/2
x = (3/2) + y

x+3y = -7
x+y= -7/3
x= (-7/3) -y
2. x= 2
    y= -3

4. x= 8.5
    y= 4.7