|re: "Just formula with comma didn't worked (may be because it is excel 2007)"|
I don't think that it is because you're running Excel 2007. Rather I believe it is due to either your Excel language version and/or Windows Regional settings.
The default separator for all US-English based Excel versions in the US in a comma.
P.S. Just be aware that if you are using the following formula, you are going to get 6000 for any combination of values in A3 and A4 other than the three specifically used in the formula.