"denote the column numbers in which the leadingentries 1 occur"
That's what you got so far - it is column 2,4 and 5
There are three columns remaining, right?
In the essay: "the R E M A I N I N G column numbers being denoted by "
[there are 6 columns, so every = a different column number , i =1,2,3,4,5,6]
Do you see it now?