I don't 'know' the answer, but I know how to find it.
28 = 3m - 7m
Look at the right side. If it said " 3 cows - 7 cows ", you'd know right away
that they add up to - 4 cows .
Similarly, 3m - 7m = - 4m
So now you have 28 = -4 m
Divide each side by -4 : -7 = m
The 1,001th time is always a charm.
