Problem On Query Operation

Hello, I have my table Produits :

CREATE TABLE [dbo].[Produit] (
[Produit_ID] [int] IDENTITY (1, 1) NOT NULL ,
[Reference] [nvarchar] (50) COLLATE French_CI_AS NULL ,
[Designation] [nvarchar] (50) COLLATE French_CI_AS NULL ,
[Quantite] [int] NULL ,
[PrixU] [sql_variant] NULL ,
[MontantHT] [sql_variant] NULL ,
[TVA] [sql_variant] NULL ,
[Facture_ID] [int] NOT NULL
)
GO
here is stored procedure CREATE PROC spBaseTVA_Bis
(
@Facture_ID int

)
AS

SELECT SUM(MontantHT) AS montantHT, TVA

FROM

Produit

GROUP BY TVA, Facture_ID
HAVING ( Facture_ID = @Facture_ID)
GO

My stored procedure  fill data in a datagrid with to column  TauxTVA and TVA  like this :
    TauxTVA      TVA
 
But I would like to add a theard column as (the value of the fisrt colum) *  ( the value of the second colum )
    
I would like to desplay data like this  :
    
     TauxTVA      TVA    Prod
       X           Y     X*Y
      
How can I modify my stored procedure to perform this ?
    
Regards