[1 3 1 -1] [1 3 1 -1] [1 3 1 -1] [1 3 0 -4]
[2 6 -1 -11] (R1X(-2)+R2-->R2) [0 0 -3 -9] (R2X(-1/3)-->R2) [0 0 1 3] (R2X(-1)+R1-->R1) [0 0 1 3]
In the echelon form of the augmented matrix, the 1st and 3rd columns are pivot columns, so x2 is the free variable. We have
x1+3x2 = -4
x3 = 3,
that is,
x1 = -4 - 3x2
x2 is free
x3 = 3