Answer :

The answer is 3m. Hope it helps!
AL2006
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 = ?