Quantity A = y - x > 12 - 5 = 7 = B
--> A > B
--> A
Note: x < 5 --> -x > -5

Of course, x can be negative. And when x is negative, y - x will always be larger than y. You can pick specific number to see what happens.(12 - -1) > 12
Hope it helps.

P/s: You should know the principle as follows

A > B (A - B > 0)
C > D (C - D > 0)
--> plus 2 inequalities, we have
A - D > B - C

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let B is your benchmark.
think of the max. value for x, let say it will be 4.9999
think of the minimum value for y, let say is 12.0001

then subtract : 12.0001-4.9999= 7.0002
still A greater