Yahweh asks this rhetorical question to emphasize that others will ask the prophets what happened to their whitewash. Alternate translation: "Others will certainly say to you, 'Where is the whitewash that you put on it?'" (See: [[rc://en/ta/man/translate/figs-rquestion]])
Possible meanings are 1) that this is an honest question for which the people expect and answer or 2) this is a rhetorical question that the people ask out of sarcasm. Alternate translation: "The whitewash that you put on it did no good." (See: [[rc://en/ta/man/translate/figs-rquestion]])