(7z + 5) + (2z - 1) + (6z + 3) + (3z - 9) =
= 7z + 5 + 2z - 1 + 6z + 3 + 3z - 9 =
= 7z + 2z + 6z + 3z + 5 - 1 + 3 - 9 = 18z - 2
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