You would have no trouble at all if the problem said "11 cows - 8 cows",
or "11 books - 8 books", or "11 cats - 8 cats", or "11 footballs - 8 footballs".
So why does it scare you when the items being counted are 'm's ?
Oh ! Here's another beauty: 11 M&Ms - 8 M&Ms = ?