work on asp.net vs 05 C#.Master page header contain the bellow code

```
<script type="text/javascript" language="javascript" src="JScript.js"></script>

```

from my one page i call the javascript method 

```
 <asp:TextBox ID="txtLength" runat="server" >0</asp:TextBox>

 <asp:TextBox ID="txtHeight" runat="server" onchange="javascript:DoChange();">0</asp:TextBox>

```

javascript method is below :

```
 {
      alert("hello world");**//get this alert but don't get the bellow alert.**
     var a=document.getElementById("txtLength");
     var b=document.getElementById("txtHeight");
      alert(a.value*b.value);
   }

```

want to show value on message box .Actually want to calculate the Sft,How to ?