Expression showing forecasted stock levels

[#1]

I need to create a table chart showing our forecasted stock levels at a selected date.

The fields I have acces to is:

Part_No                     (our item number)

CurrentStock             (Our current stock level)

Transactiondate           (Date of forecasted transaction)

Transaction_type          ( Supply or demand)

Qty_supply (forecasted supply at a given date)

Qty_demand (forecasted demand at a given date)

I want to write an expression that can show me the stock levels at any given transaction date.

CurrentStock+Qty_Supply-Qty_demand

Could any body help me with this expression?

Perhaps pritty simple but I do not pretend to be an expert on this and would appriciate the help....