Adding a little procedure into a code

I am trying to line-up a little code into a code I have before, but I don't get it; the code I try to put into the other one is this one.

Sub Subtract_Row()
  Dim a
  Dim i As Long, j As Long, rws As Long
  
  With Range("E2", Range("L" & Rows.Count).End(xlUp)).Resize(, 21)
    a = .Value
    rws = UBound(a, 1)
    For i = 1 To rws
      For j = 1 To 6
        a(i, 15 + j) = Abs(a(1, j) - a(i, 8 + j))
      Next j
    Next i
    .Value = a
  End With
End Sub

and the big code (I call like that ) I am attaching my workbook.

And at what point do you want the above code to be used in the worksheet?

What problems have you been having?

Where should that code go?