Error in stored procedure

I have a stored procedure to which I pass the following parameters
@.Date smalldatetime,
@.Amount decimal(15,3)
@.Exg_Rate decimal(5,3)
Inside this stored procedure I call another one passing to it those parameters like that
EXECUTE dbo.[Opening_Balance_AfterInsert] @.Date, @.Amount*@.Exg_Rate
I receive an error at the above line saying: Incorrect syntax near '*'
if I try to put the expression between rounded brackets I receive the error Incorrect syntax near '('
How can I pass that expression?create procedure myproc
@.Date smalldatetime,
@.Amount decimal(15,3)
@.Exg_Rate decimal(5,3)
as
declare @.combo float
set @.combo = @.Amount*@.Exg_Rate
execute do.opening_balance_afterInsert @.date, @.combo
Nicksql