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Because for the first problem you would have to simplify 8 divided by 2 into 4  in which  12 - 4 = 8 then you would have to simplify 12 - 4 to 8 which 8 = 8

Because  for the second problem you would have to simplify 12 - 8 to 4 which  4 divided by 2 = 2 then you would have to simplify 4 divided by 2 to 2 which 2 = 2

There are parentheses which if there weren't you would get a different answer

Hope I helped

Sorry if I didnt

linear pair postulates