John Gill Commentary 2 Kings 10:6

"Then he wrote a letter the second time to them, saying, If ye be on my side, and if ye will hearken unto my voice, take ye the heads of the men your master`s sons, and come to me to Jezreel by to-morrow this time. Now the king`s sons, being seventy persons, were with the great men of the city, who brought them up." — 2 Kings 10:6 (ASV)

Then he wrote a letter the second time to them
Having gained his point by the former:

saying, if you be mine ,
acknowledge yourselves my subjects and servants;

and if you will hearken to my voice ;
obey my commands:

take you the heads of the men your master's sons ;
that is, take off their heads:

and come to me to Jezreel by tomorrow this time
meaning with the heads along with them:

(now the king's sons, being seventy persons, were with the great men of
the city, which brought them up; )
they were in their houses, and under their tuition, and so had an authority over them, and could dispose of them at pleasure; they were not ordinary persons to whose care they were committed, but the principal men of the city.