Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        Dim formel As String = TextBox1.Text.Replace("x", CStr( _
        ComboBox1.SelectedItem)).Replace("y", CStr(ComboBox2.SelectedItem))
        Label1.Text = MultiplicateStrings(formel.Split(Chr(Asc("*")))).ToString
    End Sub
 
    Private Function MultiplicateStrings(ByVal values As String()) As Double
        Dim loesung As Double = 1
        For Each s As String In values
            loesung *= CDbl(s)
        Next
        Return loesung
    End Function