The structure of your tblQuantity is imperfect. You should have one field, quantityChange, instead of Up and Down fields. The Change field can record both additions and subtractions, as well as the change date and any other appropriate comments (such as addition source or subtraction reason), as long as each record is only recording one transaction. This is called normalization of data.
In other words, even if on, say June 28th you have both a plus and minus transaction, you can record the plus in one record and the minus in another.
Continuing with your question. You can then make a query (qryQtySub, perhaps) as follows:
SELECT tblItems.itemID, Sum(quantityChange) AS SumOfChanges FROM tblItems INNER JOIN tblQuantity ON tblItems.itemID = tblQuantity.itemID GROUP BY tblItems.itemID;
You would then create another query that includes the two tables "as is", joined on itemID, and also include the sub-query I described above, with its itemID field also joined to tblItems.itemID.
If I were you, though, I wouldn't display the results in a query (it's too unreadable), but rather in a report, where you could add the starting quantity plus the addition of all the changes, as well as displaying the dates and any other comments. The report would have this second table as its Record Source.