private void btBack_Click(object sender, EventArgs e)
        {
            Form4_SatementOfSalary f = Form4_SatementOfSalary.GetInstance;

            f.Show();
            this.Hide();     //Hide curent window form
        }
        private void btNext_Click(object sender, EventArgs e)
        {
            //save data for pdf file
            UserInput_InvestmentTaxCredit();

            Form4_SatementOfSalary f = Form4_SatementOfSalary.GetInstance;

            f.Show();    //showing "Satement Of Salary" form
            this.Hide(); //hiding investment tex credit form
        }