Don't write code whose intention isn't obvious when you read it. You can get all sorts of results you don't expect!
And please use the
code button; it makes your posting easier to read.
Go and find a precedence chart (there is one
here) and you see that the ++ postfix operator has the highest precedence of all. The + operator has a rather lower precedence, and the assignment operators including *= have the lowest precedence of all. So you can now work out which operation is carried out first, which is carried out second and which third.
You should easily work out how you get 15. Remember 15 = 3 * 5.
When you add the round brackets () you are changing the precedence of the operators, so you can work out which operation is done first. That is more difficult to understand; I would have thought you would get 9. I shall let you explain how you get 15 and 12; remember what the difference between i++ and ++i is.