Location: Wutong Village
Petitioner: Mr. Li the Village Head
Prerequisite: Complete Cave Fight
Reward: Martial Arts Notes, 1,500 coins

One of many side quests in Wutong Village, Snake Trouble is not immediately available when you first arrive in the village. To unlock this side quest you must first complete the main quest Cave Fight, which more or less begins Yi's journey. After that you can unlock this quest by speaking to Mr. Li the Village Head, whose position in the village changes depending on the time of day. If he's not out front of the inn, he'll be near the Cave in the Mountain.

Mr. Li will tell you of an unsurprising problem: There are snakes inside the Cave in the Mountain. You probably fought several dozen during the previous quest, and Mr. Li wants you to go back in and wipe out three more. Any of the three variants of snakes will do just fine, though if you want to get this quest over with quickly you'll likely just go after the weakling Bamboo Snakes near the entrance. Mind that this is three individual snakes, not three battles.

Trek back to Mr. Li after defeating three snakes and he'll fork over a nice little reward for your not-so-hard work.