就是教也 That Is The Whole Teaching
三藏叫：「徒弟，收拾辭王。」那國王又苦留求教。行者道：「陛下，從此色欲少貪，陰功多積，凡百事將長補短，自足以祛病延年，就是教也。」三藏叫：「徒弟，收拾辞王。」那国王又苦留求教。行者道：「陛下，从此色欲少贪，阴功多积，凡百事将长补短，自足以祛病延年，就是教也。」
逐句解 Line by line
Tripitaka called, "Disciples, pack up and take leave of the king."
The pilgrims have been in this kingdom for two chapters and Tripitaka's first instruction on getting his face back is to leave. He has not enjoyed himself.
The king pressed them hard to stay and begged for instruction.
苦留求教 — pressing them to stay and asking to be taught. Having been refused an elixir he now asks the monks for doctrine, which is the first sensible request he has made in the chapter.
Pilgrim said, "Your Majesty, from now on be less greedy for the pleasures of the flesh, build up more hidden merit, and in all things make up what is short from what is long. That is enough by itself to drive out sickness and lengthen your years, and it is the whole of the teaching."
長 is cháng, length — 將長補短, using the long to patch the short, is still ordinary Chinese for balancing things out. 陰功, hidden merit, is good done where nobody sees it and no credit accrues — the opposite of a shrine with four seasons of offerings, which is what the king tried to buy Tripitaka's heart with in the chapter's third passage.
This is Wukong's one piece of doctrine in the whole chapter and it is thirty-three characters of practical advice with nothing supernatural in it: want less, do more good quietly, balance your affairs. He offers no scripture and no technique.
It is also a precise reply to everything that has gone wrong here. The king's appetite let the demon in; his willingness to buy merit publicly is what made the shrine offer possible; and the medicine he wanted was a way of not balancing anything. Wukong answers the actual case.