private void button2_Click(object sender, EventArgs e)
{
decimal kwota = decimal.Parse(textBox3.Text);
decimal kwotaVat1, kwotaVat2;
model.ObliczVat(kwota, out kwotaVat1, out kwotaVat2);
label1.Text = kwotaVat1.ToString();
label2.Text = kwotaVat2.ToString();
}