1) The passage lists the six sons that were born to David in Hebron - Amnon, Daniel, Absalom, Adonijah, Shephatiah, and Ithream and their mothers.
2) It then notes that David reigned for 7 years and 6 months in Hebron, and 33 years in Jerusalem.
3) The passage then lists 13 sons that were born to David in Jerusalem - Shimea, Shobab, Nathan, Solomon, Ibhar, Elishama, Eliphelet, Nogah, Nepheg, Japhia, Elishama, Eliada, and Eliphelet.