Look, admittedly I'm not a mathematician. I'm a single mother of two boys, and I cant work through this, nor can my son recall ever seeing it in class. We need it clearly worked out
Then your son should go to his teacher and ask about this problem. If he has been given a problem like this, he is clearly expected to be able to understand it even if not complete it. The last thing you want to do is trick his teacher into thinking he can do it so he doesn't need more instruction!