Check the value in datagridview column and if value is equal then add to textbox

Check the value in datagridview column and if value is equal then add to textbox.

    Private Sub CheckedListBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles CheckedListBox1.SelectedIndexChanged
        dt1.Clear()
        txtTaxVal3.Text = ""
        txtTaxVal12.Text = ""
        txtTaxVal18.Text = ""
        For Each item As Object In CheckedListBox1.CheckedItems
            Dim cmd As New SQLiteCommand("Select itemcode,itemname,rate,qty,subtot,uom,youch,youchdt,pono,podt,hsn,batchno,itemtype,jwino,jwidt,pid,ichallid,challdt,compname,srno,GSTRate from ChallItem where ichallid = @ichallno", con1)
            cmd.Parameters.AddWithValue("ichallno", item("ichallid"))
            Dim da As New SQLiteDataAdapter(cmd)
            da.Fill(dt1)
            dgvBillItems.DataSource = dt1
        Next
        Call lsubtot()
        Call TaxValue()

    End Sub

    Public Function TaxValue() As Decimal
        ' Dim value As String = dgvBillItems.Rows(0).Cells(20).Value.ToString()

        For i As Integer = 0 To dgvBillItems.Rows.Count() - 1


            If dgvBillItems.Rows(i).Cells(20).Value?.ToString = "3" Then
                a = 0
                b = 0
                Try
                    b = dgvBillItems.Rows.Count
                    For a = 0 To dgvBillItems.Rows.Count - 1
                        tot1 += Convert.ToDouble(dgvBillItems.Rows(a).Cells(4).Value)
                    Next a

                Catch ex As Exception
                    MsgBox(ex.Message)
                End Try

                txtTaxVal3.Text = tot1
            End If

            If dgvBillItems.Rows(i).Cells(20).Value?.ToString = "12" Then
                c = 0
                d = 0
                Try
                    d = dgvBillItems.Rows.Count
                    For c = 0 To dgvBillItems.Rows.Count - 1
                        tot2 += Convert.ToDouble(dgvBillItems.Rows(c).Cells(4).Value)
                    Next c

                Catch ex As Exception
                    MsgBox(ex.Message)
                End Try

                txtTaxVal12.Text = tot2
            End If


            If dgvBillItems.Rows(i).Cells(20).Value?.ToString = "18" Then
                e = 0
                f = 0
                Try
                    f = dgvBillItems.Rows.Count
                    For e = 0 To dgvBillItems.Rows.Count - 1
                        tot2 += Convert.ToDouble(dgvBillItems.Rows(e).Cells(4).Value)
                    Next e

                Catch ex As Exception
                    MsgBox(ex.Message)
                End Try


                txtTaxVal18.Text = tot3
            End If
        Next
    End Function