Ajax will play Twente Enschede, 1-0 winners over Utrecht on Wednesday, in the May 8 showpiece in Rotterdam.
Miralem Sulejmani set up Lorenzo Ebecilio after 13 minutes to open the scoring with a well-placed shot but Ajax gifted RKC an equaliser when Daley Blind headed into his own net.
Ajax were ahead again on 33 minutes when Sulejmani intercepted a sloppy back pass and Mounir El Hamdaoui restored the lead for Frank de Boer's side.
Two goals in quick succession from de Jong and Demy de Zeeuw early in the second half settled the match and, after Sulejmani had missed a penalty, de Jong completed the rout.
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