As he was dying, what did David charge Solomon to do?

Answer: Be strong and keep God's ways

David told Solomon to be strong, prove himself a man, and walk in the Lord's ways as written in the Law of Moses.

““I am about to go the way of all the earth. So be strong and prove yourself a man. And keep the charge of the LORD your God to walk in His ways and to keep His statutes, commandments, ordinances, and decrees, as written in the Law of Moses, so that you may prosper in all you do and wherever you turn,”— 1 Kings 2:2-3 (BSB)