John Gill Commentary 2 Chronicles 12:13

"So king Rehoboam strengthened himself in Jerusalem, and reigned: for Rehoboam was forty and one years old when he began to reign, and he reigned seventeen years in Jerusalem, the city which Jehovah had chosen out of all the tribes of Israel, to put his name there: and his mother`s name was Naamah the Ammonitess." — 2 Chronicles 12:13 (ASV)

So Rehoboam strengthened himself in Jerusalem
Fortified it yet more for the defence of himself, after Shishak departed:

and reigned ;
twelve years more, for he reigned in all seventeen, and this was in his fifth year; of what follows in this verse, (See Gill on 1 Kings 14:21).