What was the relationship between Methuselah and Noah?

Methuselah was Noah’s grandfather: his son Lamech was the father of Noah. Genesis 5:25, 28-29

What the Bible says

Genesis 5:25

And Mathusala lived a hundred and eighty-seven years, and begot Lamech.

Genesis 5:29

And he called his name Noe, saying: This same shall comfort us from the works and labours of our hands on the earth, which the Lord hath cursed.

Douay-Rheims Bible (Challoner revision), public domain.