Your 10 point armor piece will reduce dragon's bite damage by 3%. If his bite does 50 damage, the damage you take will be 48.5
Your 10% fire resistance potion will reduce the damage from his fire breath by 10%. If his fire breath does 50 damage, the damage you take will be 40.
So, if you have the armor and the potion, you should probably wear the armor and drink the potion. It may or may not help you survive, but it's most likely better than nothing.