Store Procedure not Return calculated result

Question

sir 

 

ALTER PROCEDURE [dbo].[GL_IntrestCalculation]

@bal_amt numeric(10,2),

@intrst_rate decimal(6,3),

@dueDateCount decimal(6,3) ,

@result DECIMAL(5,3) out

AS

BEGIN

Declare @str nvarchar(MAX);

SET @str = '@bal_amt + @intrst_rate  + @dueDateCount / 360'

select cast(@str AS decimal(5,3))

set @result=@str

return (@result)

END.

 

here throgh program am passing  @bal_amt,@intrst_rate,@dueDateCount these three values and after that i want to return the @result answer to the textbox... help me sir