Add Calculated Fields

I have 2 tables with the following SQL statements:
_WorkOrder
select `ID`,

`WOID`,

`CustomerName`,

`Hours`,

`Rate`,

`Material`,

RateHours as LaborTotal,

Material0.05 as Tax,

`WorkDone`

From `_WorkOrder`
_Invoice
select `InvoiceID` ,

_WorkOrder.CustomerName,

_WorkOrder.Hours,

_WorkOrder.Rate,

_WorkOrder.Material,

_WorkOrder.WorkDone

From `_Invoice`

inner join _WorkOrder on _Invoice.WOID=_Invoice.WOID
I am trying to get a total in the invoice table for basically LaborTotal+Material+Tax as InvoiceTotal
LaborTotal and Tax are both calculated fields in the table _WorkOrder. How do I add them together?
kw