The problem here - and it's one I see a lot - is that isn't that teacher correct about the child "understanding collecting like terms"? - as they have correctly demonstrated it. But what they've also demonstrated (assuming they got beyond stage 2) is that they don't understand either subtraction, directed number of both. And that's something I see time and time again. Kids we are teaching "higher level skills" to who can and will be able to use those "skills" to hoover up some, perhaps most of the marks on a question of that type in GCSE, but who don't really have the "skill" because they still don't get directed number I'm increasingly aware that those kids I see who "get algebra" are those who "got" directed number, order of operations and at the highest level, fractions. Those who "don't get algebra" actually don't get directed number or order of operations.