Excel Application in VB

hi, below is a Sub that works with Excel. Now, I don't have problem writing details into Excel file. I would like to do some calculation on Adjustment, CurrentCharges and TaxAmount after all details (like using sum function). How to do that?

Thanks.

Public Sub tryExcel()
    Dim i As Integer
    Dim strSQL As String
    Dim xlApp As Excel.Application
    Dim xlBook As Excel.Workbook
    Dim xlSheet As Excel.Worksheet
   
    Set xlApp = CreateObject("Excel.Application")
    Set xlBook = xlApp.Workbooks.Add
    Set xlSheet = xlBook.Worksheets(1)
   
    xlSheet.Cells(1, 1).Value = "Invoice Summary Report"
    xlSheet.Cells(2, 1).Value = "Date: {09/01/2002}"
    xlSheet.Cells(4, 1).Value = "Account No"
    xlSheet.Cells(4, 2).Value = "Invoice No"
    xlSheet.Cells(4, 3).Value = "Adjustment (RM)"
    xlSheet.Cells(4, 4).Value = "Current Charges (RM)"
    xlSheet.Cells(4, 5).Value = "Govt. Tax (RM)"
    xlSheet.Cells(4, 6).Value = "Total Amount (RM)"
   
    strSQL = "Select * From aInvoice_SummaryReport Order By InvoiceNo Asc"
   
    Call OpenRS(Rs, strSQL)
    If Not (Rs.BOF Or Rs.EOF) Then
        nTotalRec = Rs.RecordCount
        For i = 0 To nTotalRec - 1 Step 1
            xlSheet.Cells(i + 5, 1).Value = Rs.Fields("AccNo")
            xlSheet.Cells(i + 5, 2).Value = Rs.Fields("InvoiceNo")
            xlSheet.Cells(i + 5, 3).Value = Rs.Fields("Adjustment")
            xlSheet.Cells(i + 5, 4).Value = Rs.Fields("CurrentCharges")
            xlSheet.Cells(i + 5, 5).Value = Rs.Fields("TaxAmount")
            xlSheet.Cells(i + 5, 6).Value = Rs.Fields("Adjustment") + _
                                        Rs.Fields("CurrentCharges") + _
                                        Rs.Fields("TaxAmount")
            Rs.MoveNext
        Next i
    End If
    Call CloseRS(Rs)
    xlSheet.Cells(nTotalRec + 5 + 1, 2).Value = "Grand Total"
   
    'xlSheet.Cells(nTotalRec + 5 + 1, 6).Value
    xlSheet.SaveAs CurrentDirectory & "\Test.xls"
    xlSheet.Application.Quit
    Set xlBook = Nothing
    Set xlApp = Nothing
End Sub