Solution
For this case we have the following system of equations:
3x-y =-1 (1)
x +y= 13 (2)
Solving x from the (2) equation we have:
x=13-y (3)
Replacing (3) into (1) we got:
3(13-y) -y= -1
Solving for y we have:
39 -3y -y = -1
4y = 40
y= 10
And solving for x we got:
x= 13-10 = 3
