Option Explicit

Sub amritraj()
    With UserForm1
        .wow.Value = True
        With .Comb
            .AddItem "addition"
            .AddItem "subtraction"
        End With
        .Comb.Value = "subtraction"
        .TextBox1.Value = 20
        .TextBox2.Value = 30
        .TextBox3.Value = 40
        .Show
    End With
    Unload UserForm1
End Sub

Option Explicit

Private Sub cancel_Click()
    Hide
End Sub
Private Sub ok_Click()
Dim a As Variant, b As Variant, c As Variant
    If TextBox1.Value = "" Then
        MsgBox "a field is empty"
        Exit Sub
    End If
    If TextBox2.Value = "" Then
        MsgBox "b field is empty"
        Exit Sub
    End If
    If TextBox3.Value = "" Then
        MsgBox "c field is empty"
        Exit Sub
    End If
    a = TextBox1.Value
    b = TextBox2.Value
    c = TextBox3.Value

    If wow.Value = True Then
        Select Case Comb.Value
            Case "addition"
                Call sum(a, b, c)
            Case "subtraction"
                Call minus(a, b, c)
        End Select
    End If
    Hide
End Sub

Sub sum(a As Variant, b As Variant, c As Variant)
    Cells(1, 1).Value = Application.WorksheetFunction.sum(a, b, c)
End Sub
Sub minus(a As Variant, b As Variant, c As Variant)
    Cells(1, 1).Value = Application.WorksheetFunction.sum(a - b - c)
End Sub