Gaussian Elimination means that the augmented matrix should be reduced to a triangular matrix, and then use reverse substitutions to solve. The matrices look like this:
[1 1 1 9]
[1 -2 3 8]
[2 1 -1 3], then
[1 1 1 9]
[0 -3 2 -1]
[0 -1 -3 -15], then
[1 1 1 9]
[0 1 3 15]
[0 -3 2 -1], then
[1 1 1 9]
[0 1 3 15]
[0 0 11 44].
The last row stands for the equation 11z = 44, so z = 4.
The second row stands for the equation y + 3z = 15, so y + 3(4) = 15, and y = 3.
The first row stands for x + y + z = 9, so x + 3 + 4 = 9, and x = 2.
[1 1 1 9]
[1 -2 3 8]
[2 1 -1 3], then
[1 1 1 9]
[0 -3 2 -1]
[0 -1 -3 -15], then
[1 1 1 9]
[0 1 3 15]
[0 -3 2 -1], then
[1 1 1 9]
[0 1 3 15]
[0 0 11 44].
The last row stands for the equation 11z = 44, so z = 4.
The second row stands for the equation y + 3z = 15, so y + 3(4) = 15, and y = 3.
The first row stands for x + y + z = 9, so x + 3 + 4 = 9, and x = 2.