My last period was 7.7.12 i had unprotected sex on the 15.7.12 and also on the 28.7.12. i have 24 day cycle what is my date of conception ?

The second: Assuming everything is working normally with your physiology, the 7/28 date is most likely.

Conception date: Generally, you have ovulation near midcycle counting from the first day of your last period. So if a 24 day cycle, you would ovulate about day 12. Your fertile period would begin a few days before that. So, my guess would be the earlier date, because the 28th of the mounth is near the end of your 28 day cycle.