Clara simplifies (8b-4r) - (2b-r) and says the result is 6b-4r. What error did Clara make?

Hello! The error that Clara made was that she didn't combine the r's together. For questions like these, we would combine like terms. The 6b part is correct, because 8b - 2b is 6b. But, the other r needs to be combined with the - 4r, and in this case we would add, because subtracting a negative number turns the problem from subtracting to adding. - 4r - (-r) is - 3r. The correct result is really 6b - 3r.