Use the trigonometric function.

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 = ?
I believe looking at a weird perspective the answer would be 12 females. :)
Answer:
4x - 12 = 28
To isolate 4x, we add 12 to both sides:
4x - 12 + 12 = 28 + 12
so
4x = 40
Now we divide 4 from both sides:
x = 40 / 4
so x = 10