Hello adkinsjr.

I think the answer is in the text?

"denote the column numbers in which the leadingentries 1 occur"

That's what you got so far - it is column 2,4 and 5

so and

There are three columns remaining, right?

So

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?

Yours

Rapha