שַׁלּ֤וּם בֶּן־יָבֵישׁ֙ מָלַ֔ךְ בִּשְׁנַ֨ת שְׁלֹשִׁ֤ים וָתֵ֙שַׁע֙ שָׁנָ֔ה לְעֻזִיָּ֖ה מֶ֣לֶךְ יְהוּדָ֑ה וַיִּמְלֹ֥ךְ יֶֽרַח־יָמִ֖ים בְּשֹׁמְרֽוֹן׃
Metzudat David on II Kings
Because Zechariah reigned for 38 years during Uziah's reign, and he reigned for 6 more months, and Shalum reigned during the 39th year of Uziah.