What advice did the elders give Rehoboam?

Answer: Serve them kindly

The elders said that if Rehoboam would serve the people and speak kind words, they would be his servants forever.

“They replied, “If you will be a servant to these people and serve them this day, and if you will respond by speaking kind words to them, they will be your servants forever.””— 1 Kings 12:7 (BSB)