Is there some sort of rule for subtracting the two systems of equations? I vaguely remember it being mentioned in the prepswift video, but am more used to the traditional ways of solving 2 equations (i.e. substitution or addition/subtraction). I did it via the shortcut but don’t know the rationale…
I think just observing that x and y are swapped around in the second one, and from that figuring out if you subtract them you can get a coefficient of 4 and -4 for each. I don’t think there was a rule but you can just keep this trick in mind for future questions.