getting error as converting data type nvarchar to numeric.

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C#

i have table answer with colums as answer_id primary key,student_id foreign key,question_id foreign key,student_answer varchar(400)
and question table contains colums as question_id primary key,teacher_id foreign key,tea_que varchar(400)
and table calculation contains colum student_id,marks numeric
i am trying insert student_id,question_id,student_answer in answer table which is executing properly.
after i am comparing teacher's and students's answer and calculating score and storing in label or variable.
at same time i am trying to insert/take values student_id and calculated marks in calculation table
code is:

 public partial class WebForm9 : System.Web.UI.Page
    {
       
        SqlCommand cmd = new SqlCommand();
     
       DataTable dt = new DataTable();
        DataRow drr;
        public string ans_tea;
        public string ans_stu;
        SqlConnection con = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=swatidb;Integrated Security=True;Pooling=False");

      protected void insert_Click(object sender, EventArgs e)
       {
           try
           {
               String que_id = Label3.Text;
               // String que = Label2.Text;
               String stu_id = Label4.Text;
               String ans = TextBox1.Text;
            cmd = new SqlCommand("insert into Answer values(@que_id,@stu_id,@ans)", con);
               cmd.Parameters.AddWithValue("@que_id", que_id);
               cmd.Parameters.AddWithValue("@stu_id", stu_id);
               cmd.Parameters.AddWithValue("@ans", ans);
               int i = cmd.ExecuteNonQuery();
               using (SqlConnection con1 = new SqlConnection(@"Data Source=.\sqlexpress;Initial Catalog=swatidb;Integrated Security=True;Pooling=False"))
               {
                   int qu_i = Convert.ToInt32(Label3.Text);
                   con1.Open();
                   using (SqlCommand cmd1 = new SqlCommand("SELECT teacher_answer FROM Question WHERE question_id=@qu_i", con))
                   {
                       cmd1.Parameters.AddWithValue("@qu_i", qu_i);
                       using (SqlDataReader reader = cmd.ExecuteReader())
                       {
                           while (reader.Read())
                           {
                               ans_tea = (string)reader["teacher_answer"];
                           }
                       }
                       //Label4.Text = ans_tea;
                   }
                  int qu_id = Convert.ToInt32(Label3.Text);
                   using (SqlCommand cmd1 = new SqlCommand("select student_answer from Answer where question_id=@qu_id", con))
                   {
                       cmd1.Parameters.AddWithValue("@qu_id", qu_id);
                       using (SqlDataReader reader = cmd.ExecuteReader())
                       {
                           while (reader.Read())
                           {
                               ans_stu = (string)reader["student_answer"];
                           }
                       }
                   }
               }
               MatchsMaker match = new MatchsMaker(ans_tea, ans_stu);
               //b = match.Score;
               // Response.Write(b);
               Response.Write(match.Score);
               Label6.Text = Convert.ToString(match.Score);
               string mrk = Label6.Text;
               cmd = new SqlCommand("insert into cal_marks_of_each values(@stu_id,@mrk)", con);
               cmd.Parameters.AddWithValue("@stu_id", stu_id);
               cmd.Parameters.AddWithValue("@mrk", mrk);
               int j = cmd.ExecuteNonQuery();
               TextBox1.Text = "";
           }
           catch (Exception e1)
           {
               Label5.Text = "values are not inserted because you have alredy inserted question"+e1;
           }
       }
}
}