Here’s a common mistake, and a very understandable one, too. Students need to combine two negative numbers, and they, of course, wind up with an answer that’s positive. Why? Because, they’ll say — pointing out that you yourself have told them this — “Two negatives make a positive!”
This video gets to the root of this common misunderstanding by helping students understand exactly when two negatives make a positive, and when they don’t.
Make sure you watch the whole video, as there are practice problems at the end, along with their answers.