Your answer, "22", is not correct.
The correct answer to
How old was Ahaziah when he began to reign?
is
42
as shown by
2 Chronicles 22:2
which is
- KJV
- Forty and two years old was Ahaziah when he began to reign, and he reigned one year in Jerusalem. His mother's name also was Athaliah the daughter of Omri.
- NIV
- Ahaziah was twenty-two {Some Septuagint manuscripts and Syriac (see also 2 Kings 8:26); Hebrew forty-two} years old when he became king, and he reigned in Jerusalem one year. His mother's name was Athaliah, a granddaughter of Omri.