Determine if the matrix is in row-echelon form. (the following numbers are all in brackets) 1 0 0 0| 2 0 1 0 0| 4 0 0 -1 0| 9 0 0 0 1| -6 Please...

Determine if the matrix is in row-echelon form. (the following numbers are all in brackets)

1 0 0 0| 2

0 1 0 0| 4

0 0 -1 0| 9

0 0 0 1| -6

Please explain why this is not in row-echelon form. I think I understand how to reduce and equation, but not sure how to determine if this is in row-echelon form just based on looking at it.