Q: WHAT YEAR DID AHAZIAH BEGIN HIS REIGN IN JUDAH?
A: IT DEPENDS WHAT CALENDAR YOU ARE USING!
On the Gregorian calendar (the one you might have on your wall) the year was 839 BC.
On the Hebrew calendar, sometimes called the biblical calendar, the year was 3208 AM.
I Kings 22:51 - Ahaziah the son of Ahab began to reign over Israel in Samaria the seventeenth year of Jehoshaphat king of Judah, and reigned two years over Israel.