Question: When did Nguyễn Phúc Miên Bửu's father die?
Sentence (from "Trương Phúc Loan"): Hearing the news, Trịnh Sâm, the lord of northern Vietnam, sent Hoàng Ngũ Phúc marched south in 1774, using the excuse that they would help Nguyễn lord to arrest the treacherous minister Trương Phúc Loan.
Relevant evidence: No