Or, just give the c field a formula of ([a] + [b]):
CREATE TABLE [tblTable] (
[IDField] [int] IDENTITY (1, 1) NOT NULL ,
[a] [int] NULL ,
[b] [int] NULL ,
**[c] AS ([a] + [b])** ,
CONSTRAINT [PK_tblIDField] PRIMARY KEY CLUSTERED
(
[IDField]
) ON [PRIMARY]
) ON [PRIMARY]
--EricDV Sig--------- Some problems are so complex that you have to be highly intelligent and well informed just to be undecided about them. - Laurence J. Peters