"A" - "B" is the total not on the scale, so (A-B)-C = Pin weight. The pin weight + trailer weight = gross trailer weight.
21540-10420=11120 not on the scale: 11120-8860=2260, pin weight: 2260+10420=12680: 12680-12120=560
I think that's right, if not, someone will be along and get me straight.
This is assuming the truck was not disconnected which is the way I did it.