Create a query via the database and insert the required table
Select the Records_T.Amount field into the query grid.
On the tool bar, click on the totals symbol (the sigma or Greek S) and check that the totals row appears in the query grid.
Select sum from the drop down list for the field "Amount" in this row.
Run the query and it will add up both the negative and positive values to give an overall sum
You can of course filter this for values greater than or less than 0 via the criteria.
Hope this helps