There seems to be a mistake in the mark scheme: it says (x+y)^2 when it should say (x-y)^2.
Also, even if that were correct, there is another mistake in that it says:
x(x−y)−y(x + y)−(x + y)^2 = −2y^2 , which I'm fairly sure isn't true.
Could someone please take a look at this and make any necessary changes (assuming I'm not being stupid, which is, to be fair, quite likely)