If the villain has a Kx, he certainly played it in a weird way, most people wouldve shown aggression much sooner with this type of a hand. This looks like to me that he called down with 99s hoping his 2 pair was best and then he hit his magic card on the river. Going by sheer mathematics, you should just call and heres my reasoning. If you assume the villain will raise AK,KQ,KJs,and 99 utg then you are only a 5-3 favorite(this also assumes the villain is not an idiot and would not play AQ,AA,QQ,JJ,TT this way) being a 5-3 favorite is not enough to raise this river since you are laying 2-1 on your raise becuz you must call a 3 bet. Going by logic, you should also just call since most people do not play trip kings this way, pocket 99's is much more logical than trip kings given this action. So final answer is call the river checkraise.