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.
