Base your forms recordsource on a query ( in property sheet, click the elipsis to start the query builder)
Add an entry X1 + X2. the sql will look like this
Code:
SELECT A.id, A.x1, A.x2, A.name, A.xx, A.a, A.b, [x1]+[x2] AS Expr1
FROM A;
for the a\b check boxes you could either use an option group or in the after update events of the checkboxes use
Code:
Private Sub a_AfterUpdate()
If Me.a = 0 Then
Me.b = -1
Else
Me.b = 0
End If
End Sub
Private Sub b_AfterUpdate()
If Me.b = 0 Then
Me.a = -1
Else
Me.a = 0
End If
End Sub