Simple arithmetic between 2 columns not in the same table

[birdleaf]

Hi,

I am simplifying to make things easier. I have App A with Table Inventory. Within Table Inventory, I have columns "ID", "Subtotal" and "Total". I use App A to enter inventory. I also have App B with Table Tracking. Within Table Tracking I have columns "ID", and "Subtotal". I use App B to enter tracking. Based on what I enter in App B, I would like "Total" within App A to automatically update. This is simple arithmetic of the following: Table Inventory[SubTotal] - Table Tracking [Total]. How would I go about doing this? When I put the above formula in the Autopopulate section of that column, I am getting the following error:

"...Arithmetic expression has input of type unknown". Both columns are of type Number so I am guessing that is not what this error is referring to. Help appreciated. Thanks!

[Trevwiller]

In the current expression you are subtracting a list of items from a list of items. You must first get the sum of both lists to subract from eachother. SUM(Table Inventory[SubTotal]) - SUM(Table Tracking [Total])

[birdleaf]

I wasn't clear enough, I would like the sum of each row, so not the sum of the entire column minus the sum of the other column. 

[AlirioP]

Hello! Could you please provide a screenshot? I think your arithmetic operation is using the entire list of both tables.