作善可解 Goodness Is the Only Key
此乃上天所設之計，你怎麼得見？」三藏道：「似這等說，怎生是好？」行者道：「不難，不難。我臨行時，四天師曾對我言，但只作善可解。」此乃上天所设之计，你怎么得见？」三藏道：「似这等说，怎生是好？」行者道：「不难，不难。我临行时，四天师曾对我言，但只作善可解。」
逐句解 Line by line
"This is a device set up by Heaven above. How would you even get to see it?"
得 is dé here, following the interrogative 怎麼 — how would you manage to. 計 is a scheme or a contrivance, and calling heaven's arrangement a device is a shade cooler than reverent.
Tripitaka said, "If that is how it is, then what is to be done?"
似 is sì, always this reading in this edition. 怎生 is an older equivalent of the modern how — the same word the novel uses whenever a character has run out of ideas.
Pilgrim said, "Not hard, not hard.
The doubled reassurance, which Pilgrim has used twice already in this chapter. He said the same thing about praying for rain, before he found out it required an edict.
"When I was leaving, the Four Preceptors told me that only the doing of good can undo it."
曾 is céng, the past marker — once, formerly. 作善可解, doing good can undo it, arrives now for the second time, and this time it is being said to the man it applies to.
Pilgrim swats Bajie down, Tripitaka asks the useful question, and the answer that has been sitting in Pilgrim's pocket since heaven finally comes out.
The sequence matters. He was told this before he came down, and he has spent the intervening scene shouting at the prefect, extracting a confession, and reciting the punishment in detail. Only now, prompted by his master, does he mention that there is a way out.
Not hard, he says — the same two words he used before the whole thing turned out to be very hard indeed. This time he happens to be right.