I believe you want to select a subset of rows and subset of columns of your
original matrix m.
If you had wanted only the first row of m, you could have used m[1,]
Alternatively, if you had wanted only the second column of m then you could
have used m[,2]
m[1,2] would give you the element at row 1, column 2.
You are requesting rows 2,7, and 8 and columns 1,4,6 and 7.
The syntax is m[required rows, required columns]
c() allows you to specify multiple rows/columns at the same time.
HTH
Pete