private void FrmHasil_OnPerhitungan(int index, float a, float b)
        {
            var cal = new Calculator();

            if (index == 0)
            {
                lstHasil.Items.Add(string.Format("Hasil Penjumlahan {0} + {1} = {2}", a, b, cal.Penjumlahan(a, b)));
            }
            else if (index == 1)
            {
                lstHasil.Items.Add(string.Format("Hasil Pengurangan {0} - {1} = {2}", a, b, cal.Pengurangan(a, b)));
            }
            else if (index == 2)
            {
                lstHasil.Items.Add(string.Format("Hasil Perkalian {0} * {1} = {2}", a, b, cal.Perkalian(a, b)));
            }
            else
            {
                lstHasil.Items.Add(string.Format("Hasil Pembagian {0} / {1} = {2}", a, b, cal.Pembagian(a, b)));
            }
        }