Calculate cubic root

I'm having trouble creating the following Cube root and LN buttons. I have buttons that should be very similar like for example the normal square root and the log.

I tried to create the following for the cube root:

   private void btn_raizcub_Click(object sender, RoutedEventArgs e)
    {
        primeiro += double.Parse(janela.Text);
        resutado = Convert.ToSingle(Math.Sqrt(primeiro/3.0));
        janela.Text = resutado.ToString();
    }