Something similar to this?
CREATE TRIGGER dbo.trgMyTotalUpdate ON dbo.MyTable
AFTER UPDATE
AS
IF UPDATE(Quantity)
UPDATEmt
SETmt.Total = i.Quantity * i.Price
FROMdbo.MyTable AS mt
INNER JOINinserted AS i ON i.PkCol = mt.PkCol
N 56°04'39.16"
E 12°55'05.25"