@Laura_Ramazzotti, a table in AppSheet does not have an order, unlike a spreadsheet. So there is no notion of “previous”. Instead, you will want to identify the most recent row by using the MAXROW() function in a virtual column.
See help.appsheet.com - List Expressions and Aggregates and look for MAXROW().
In your case, you will need a virtual column called MostRecentRow with app formula MAXROW(“Account”, “date”) And then for the balance, use this app formula: [MostRecentRow].[balance] + [income] - [expenses] List Expressions and Aggregates help.appsheet.com