Just tell yourself that theres absolutely nothing you can do about it now, so theres no point in freaking out. Also, try to remind yourself exactly how big a difference those 2 or 3 marks will make in the long run anway.
Then, instead of freaking out that you "don't know" the exact answer, try to remember everything you do know about it, and put yourself in the merkers head, and think about the best way to attempt it that they may swing you at least partial credit. Also think back to your classes when you may have learned it, instead of whatyou actually sat down and memorised it...something a teacher said may just come back to you.