From the above you need to have a column that has the item that you want to aggregate with. In your example you have [Customers: items] which has the customer name and the item name together. You need to separate the item name from the customer first.
Secondly, to have a total count of the quantity and assigned you can then run a group by query, e.g.
select item, sum(quantity_store) as qty, sum(assign) as out from stock group by item;
this will give you a qty of 36 and out of 10 per item in stock
Unfortunately your question is still very unclear however at column level, you can run a query like this
select item, sum(quantity_store) as qty, sum(assign) as out, (quantity_store - assign) as instock from stock group by item;
this returns the difference between quantity_store and assign. Hope it helps somehow.