Sheetal,
The first thing to do is to use the Excel sheet as a linked table (File >Get
External Data > Link Table).
Then you make a query on it where you can read the values for x and y, as
well as add the two in a caclulated field.
You can then make this query an append or update one (Query > ...., depends
on what you want to do) to store the information in a table, if required
(under certain conditions it may not).
Wheter you store the x's and y's separately or just the sum, depends on what
data you'll need to recall!
HTH,
Nikos