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-5a + 6(a+c) - 4c = -5a + 6a + 6c - 4c = a + 2c.
It is your ANSWER.
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This quasi- and pseudo- tutor @josgarithmetic is given to us as a kind of punishment at this forum.
It is a punishment for all "students" / (visitors), as well as for all tutors.
Every day a team of cleaners should work to clean after him.