Charles Ellicott Commentary 2 Kings 14:17

"And Amaziah the son of Joash king of Judah lived after the death of Jehoash son of Jehoahaz king of Israel fifteen years." — 2 Kings 14:17 (ASV)

Fifteen years. —He came to the throne in the second year of Jehoash, who reigned sixteen years (2 Kings 13:10), and reigned twenty-nine years (2 Kings 14:2). The different data are thus self-consistent. Jehoash appears to have died very soon after his victory—perhaps in the following year.