2 Kings 18:1 - The Popular Commentary by Paul E. Kretzmann

Now, It came to pass in the third year of Hoshea, son of Elah, king of Israel, that Hezekiah, the son of Ahaz, king of Judah, began to reign. The apparent difficulty with regard to the dates at this period is readily solved by remembering that the years of the two kings do not run exactly parallel. "If we assume that Hezekiah's accession took place near the end of Hoshea's third year, then his fourth and sixth years correspond, for the most part, with the sixth and ninth of Hoshea. " (Keil.)