Answer:
3rd. row becomes [0 0 -6 1]
Step-by-step explanation:
The element of pivot is the 9 in the 2nd row, so the 1st and 2nd rows stays the same.
For the 3rd row we have to calculate in each column, the element of the 3rd row minus the element of the 2nd row, and put it in the 3rd row.
This way we have <em>four operations</em> as follow:
for columns 1 to 4 in that order.
With this procedure, the system becomes
[1 –2 3 1]
[0 9 2 –5]
[0 0 –6 1]