Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Button1.Click
        Dim objexcel
        objexcel = CreateObject("Excel.application")
        objexcel.Visible = True
 
        objexcel.Workbooks.add()
 
        objexcel.Range("B3").Select()
        objexcel.ActiveCell.Value = ComboBox5.Text
 
        objexcel.Range("C3").Select()
        objexcel.ActiveCell.Value = ComboBox7.Text
 
        objexcel.Range("D3").Select()
        objexcel.ActiveCell.FormulaR1C1 = "=RC[-2]+RC[-1]"
 
        objexcel.Quit()
        objexcel = Nothing
End Sub

'In Excel
        objexcel.Range("D3").Select()
     	objexcel.ActiveCell.FormulaR1C1 = "=RC[-2]+RC[-1]"