A quick recap is necessary now:

Methuselah was 369 years old when Noah,his grandson was born.

Noah was the one with the responsibility of building the ark to save humanity from the deluge that Enoch saw ahead that made him to name his son Methuselah.

Are you getting it?

Now,how old was Noah when the flood was upon the earth?

“Noah was 600 years old when the flood covered the earth.”-Gen 7:6

This,therefore,confirmed the age of Methuselah of 969 years old that is 369 years when he had Noah plus 600th birthday of his grandson,Noah.

“Altogether, Methuselah lived a total of 969 years, and then he died.”- Gen 5:27

Methuselah’s year of death was precisely the time flood(deluge) came over the earth as prophesied by Enoch,his father.