John Gill Commentary 2 Kings 15:17

"In the nine and thirtieth year of Azariah king of Judah began Menahem the son of Gadi to reign over Israel, [and reigned] ten years in Samaria." — 2 Kings 15:17 (ASV)

In the nine and thirtieth year of Azariah king of Judah began
Menahem the son of Gadi to reign over Israel
Shallum reigning but one month, both their reigns began the same year.