I though that A and C had to be greater than 0, and if they are a negative aren't you supposed to multiply by -1? Am i right or wrong?

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Did you see the video clip? The answer in the clip is 2x + y = -5, which can be represented by Ax + By = C. In this case, A = 2 and B = 1 making BOTH A and B > 1. In the form Ax + By = C, A cannot be negative. So, if A is negative, you can multiply across by -1 to convert A to positive.