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In Access you can create formulas (called 'expressions' in Access) in tables, queries, forms, reports, and macros. If you want to apply an expression in a table it has to be done for the 'field' which will apply to all records, but you cannot apply it to one cell only (i.e. for one record only). Also expressions in Access (and Excel) start with the equal sign and not the @ symbol. So for the sum formula that you typed, it would be =Sum(1200-2400)
Hope this helps. Let me know if you have any questions.