how to find index of item's in datalist

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C#

LINQ

C#4.0

ASP.NET4

i have a datalist i want to find out index value of the cell in which book_id==73
and want to show some labels text in dt cell
this is my datalist

<asp:DataList ID="dl_cartitems" runat="server" 
                        onitemcommand="dl_cartitems_ItemCommand">
                  <ItemTemplate>
                  <tr bgcolor="#FFFFFF" class=" product-detail" >
                    <td width="14%" valign="top"> <img src='<%#"book_image/"+ Eval("Image") %>' width="128px" style="margin-top:10px;margin-right:10px;margin-left:10px;margin-bottom:10px" /></td>
                    <td width="43%"  valign="top" style="padding-removed10px">The Kite Runner by Khalid Hosseini <br /> <asp:Label ID="discount_message" runat="server"></asp:Label> </td>
                    <td  width="6%" align="center" valign="top" style="padding-removed10px"><a href="#">Edit</a></td>
                    <td   width="10%" align="center" valign="top" style="padding-removed10px"> $<asp:Label ID="lbl_parprice" runat="server" Text='<%#Eval("Price") %>'></asp:Label></td>
                    <td width="10%" align="center" valign="top" style="padding-removed10px">
                     <asp:DropDownList ID="ddl_quantity" runat="server" Width="60px" OnSelectedIndexChanged="ddl_quantity_selectedindexchanged" AutoPostBack="true" >
                    <asp:ListItem> 1 </asp:ListItem>
                    <asp:ListItem> 2 </asp:ListItem>
                    </asp:DropDownList> </td>
                    <td width="12%" align="center" valign="top" style="padding-removed10px">$<asp:Label ID="lbl_subtoprice" runat="server" Text='<%#Eval("Price") %>' ></asp:Label> </td>
                    <td width="5%" align="center" valign="top" style="padding-removed10px" ><asp:ImageButton ID="imgbtn_del"  ImageUrl="~/images/del.jpg" runat="server" CommandName="Delete" CommandArgument='<%#Eval("id") %>' ></asp:ImageButton></td>
                  </tr>
                  </ItemTemplate>
                  </asp:DataList>

this is my datalist now i want to find cell in which book_id is 73 and want to show some text in that cell's label
below code

<asp:Label ID="discount_message" runat="server"></asp:Label>

thanks