The problem
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On the 2nd line why did you combine the 5y and 7y, but on the 3rd line you bring them back and reverse them. Words to explain would be very helpful.
Answer provided by our tutors
Our system works in such a way that it first simplifies the expression (that means combining all the terms) and then tries to factor. It turned out that in this case it was unnecessary (because the factoring process split up the terms in the exact same way), but in 99% of problems simplification has to be done first (e.g. imagine you had 2x-6x - you would have to simplify first).
So, in this case just ignore the original simplification - it wasn't really necessary
NJ