create first a Query for your table that you use in your Subform, eg:
(qrySubData)
select * from yourSubFormTableName where BrNumber = IIF(IsNull([Forms]![frmEstimateBt]![cboBrNumber]), [BrNumber], [Forms]![frmEstimateBt]![cboBrNumber])
In design view of your form, use the Query (qrySubData) as the RowSource of your Subform.
add code to the AfterUpdate Event of cboBrNumber combobox:
Private Sub cboBrNumber_AfterUpdate()
Me!subfrmEstimateBr.Requery
End Sub