Need help building this expression

[Tiger1]

I have three tables in an app. I need information from two table to use in another.

Table A - needed columns:
[Sales]

Table B - Needed Colums:
[DSO]

Table C - i am using these columns.

I need to calculate:
DSO (month in sales) - Payables (same month payables are due)

Table C (So far):

Calculation (so Far - this doesnt work…)

Tried adding [Sales] from Table A - using a key (shows no data…)

I know i am not explaining it well. I just need help on this…