Does someone has any suggestions how to make 17 BTC with 7 BTC starting capital in short period of time?
No investment is guaranteed to generate you profits in a short time. The only way is gambling, or trading in BTC or in another currency (but it's basically gambling as well).
Trading is absolutely not gambling, it surely has risk as well but not as much as gambling has, and it is proven that trading needs knowledge and experience which gambling has nothing to do with.
Can you find gamblers who can proudly say that they use their experience of knowledge to make profits with it? You cannot.
But you can find a lot of traders who are earning a living with it and that they can proudly say they are traders.