Private Sub Combo1_Click()
If Text1.Text = "" Then
  MsgBox "data 1 kosong"
  Text1.SetFocus
  ElseIf IsNumeric(Text1) = False Then
  MsgBox "data 1 harus numeric"
  Text1.SetFocus
End If
   
If Text2.Text = "" Then
  MsgBox "data 2 kosong"
  Text2.SetFocus
  ElseIf IsNumeric(Text2) = False Then
  MsgBox "data 2 harus numeric"
  Text2.SetFocus
End If

    If Combo1.ListIndex = 0 Then
        Text3 = Val(Text1.Text) + Val(Text2.Text)
    End If
    If Combo1.ListIndex = 1 Then
        Text3 = Val(Text1.Text) - Val(Text2.Text)
    End If
    If Combo1.ListIndex = 2 Then
        Text3 = Val(Text1.Text) * Val(Text2.Text)
    End If
    If Combo1.ListIndex = 3 Then
      Text3 = Val(Text1) / Val(Text2)
    End If


If Text1.Text = "" Then
  Text3.Text = ""
  End If
If Text2.Text = "" Then
  Text3.Text = ""
  End If
 
If IsNumeric(Text1) = False Then
  Text3.Text = ""
  End If
If IsNumeric(Text2) = False Then
  Text3.Text = ""
  End If
 

End Sub


Private Sub Command1_Click()
Text1.Text = ""
Text2.Text = ""
Text3.Text = ""
End Sub


Private Sub Command2_Click()
Unload Me
End Sub

Private Sub Text1_Change()
Dim Text1 As Integer

End Sub

Private Sub Text2_Change()
Dim Text2 As Integer

End Sub

Private Sub Text3_Change()
Dim Text3 As Double
End Sub

http://www.ziddu.com/download/9600969/Vbkalku.rar.html