Answer:
Anything greater than 6
Sides M could both be any number greater than 6 and less than 10. The sides would have to have the same value so if one is 6 the other will also be 6. Add 10 and 6 to get 16, and that leaves 12 more units to equal the 28 that you need, and you would have to split the value by two and you would get six for M.