Holly finds that (11m – 13n + 6mn) – (10m – 7n + 3mn) = m – 20n + 9mn. What error did Holly make?

Distribute the subtraction sign to every term in the second polynomial: (11m − 13n + 6mn) − 10m + 7n − 3mn. Holly’s result, m − 20n + 9mn, shows she changed 10m to −10m but left −7n and 3mn unchanged, adding them instead. So she only used the additive inverse of 10m, which is choice A.

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