How to get the result from javascript to asp.net

I am trying to add two numbers entered in a text box with javascript call and return that to a third text box. Could any one suggest, how to show the result in third text box.
My code is

[Code]
<SCRIPT language=”JavaScript”>
function checkValue(tbox1, tbox2)
<!–
{
var tot
tot = textbox1 + textbox2;
return tot
}

//–>
</SCRIPT>

<asp:datagrid id=”MyDataGrid” runat=”server” AutoGenerateColumns=”false”>
<Columns>
<asp:TemplateColumn HeaderText=”Rent per Sq Feet”>
<ItemTemplate>
<asp:TextBox ID=”TxtRent” Runat=”server” ></asp:TextBox>
</ItemTemplate>
</asp:TemplateColumn>
<asp:TemplateColumn HeaderText=”Area” HeaderStyle-Width=”10%”>
<ItemTemplate>
<asp:TextBox ID=”TxtArea” Runat=”server” OnTextChanged=”CallFun” Text=”..” AutoPostBack=True ></asp:TextBox>
</ItemTemplate>
</asp:TemplateColumn>
<asp:TemplateColumn HeaderText=”Total” HeaderStyle-Width=”10%”>
<ItemTemplate>
<asp:TextBox ID=”TxtTot” Runat=”server” ></asp:TextBox>
</ItemTemplate>
</asp:TemplateColumn>
</Columns>
</asp:datagrid>
[/Code]

Code Behind

[Code]
Sub CallFun(ByVal sender As Object, ByVal e As EventArgs)
Dim TxtAreaInSqFeet As TextBox = sender
Dim dgItem As DataGridItem = TxtAreaInSqFeet.Parent.Parent
Dim t1 As TextBox = dgItem.FindControl(“TxtRent”)
Dim t2 As TextBox = dgItem.FindControl(“TxtArea”)
Dim t3 As TextBox = dgItem.FindControl(“TxtTot”)
t3.Attributes.Add(“onblur”, “return checkValue(‘” & t1.Text & “‘,'” & t2.Text & “‘);”)

End Sub
[/Code]

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