public void RemainingCredit()
{
int departmentId = Convert.ToInt32(DepartmentDropDownList.SelectedValue);
int teacherId = Convert.ToInt32(TeacherDropDownList.SelectedValue);
decimal CreditTaken = Convert.ToDecimal(txtCreditTaken.Text);
decimal TotalAssignCredit = _CourseAssigneManager.TotalAssignCredit(departmentId, teacherId);
decimal RemainingCredit = CreditTaken - TotalAssignCredit;
txtRemainingCredit.Text = RemainingCredit.ToString("0");
}