Mark M. answered 12/03/21
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Retired math prof. Very extensive Precalculus tutoring experience.
Recall that lAl = A if A > 0 and lA l= -A if A < 0.
If x > y, then x - y > 0. So, multiplying both sides by -1, we get -x + y < 0.
Therefore, l -x + y l = -(-x + y) = x - y