You want to add that textbox value in same(StudentRecords) table then try this:
SqlCommand cmd = new SqlCommand("INSERT into StudentRecords(FirstName,LastName,StudentName,RollNo,Result,StuRemark,TacherRemark)VALUES('" + TxtFirstName.Text + "','" + TxtLastName.Text + "','" + TxtStudentName.Text + "','" + TxtRollNo.Text + "', '" + TxtResult.Text + "','" + TxtStudentRemark.Text + "')",'" + TxtTeacherRemark.Text + "', con);