Answer: Mistake is when they added 7x to 6x and -5x and got 18x.The real answer is 8x - 11. Steps below.
Explanation:
7x + 3(2x - 4) -5x + 1 7x + 6x - 12 - 5x + 1 8x - 12 + 1 8x - 11. Can't simplify any further so this is the answer
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