I' not sure I understand your description correctly - in particular, some description of what tables / columns you have in your database and what they represent would help.
But if you are just trying to do a straightforward calculation on each record, something like the following would work.
You need to set up your data context and entities (drag and drop in the VS designer and then do a bit of renaming of relevant bits in the Properties pane is all you need). Then see code below.
If your data structure is different, post details of the data structure and I'll see what I can do to amend the code.
IEnumerable<MyCommissionStatus> MyCommissionStatuses =
(from ACommissionStatus in MyDC.CommissionStatuses
where // [add whatever you need here]
foreach(MyCommissionStatus ACommissionStatus in MyCommissionStatuses)
ACommissionStatus.commission = ACommissionStatus.NB + ACommissionStatus.Renewal - ACommissionStatus.clawback;
You might also want to think about transactions / failure modes etc. - but the above gives the basics.