when a control has runat="server" the client id is composed with all the container controls that he has.
example
my id "linkSelection" becomes "ctl00_ctl00_MasterContainer_menu_linkSelection"
you can match the final part of the id
<script type="text/javascript">
function showanswer(sender, divid) { $("[id$='"+ divid +"']").slideDown(400); }
function hideanswer(divid) {$("[id$='"+ divid +"']").slideUp(300);}
</script></script>
or you can use
<asp:TextBox ID="txt_answer" runat="server" Height="74px" TextMode="MultiLine"
Width="296px"></asp:TextBox>
<br />
<input id="Button1" type="button" value="Submit" onclick="PostAnswer('<%= div_answer.ClientID %>')" />
<input id="Button2" type="button" value="Cancel" onclick="hideanswer('<%= div_answer.ClientID %>')" />