You are correct, it would be easier to subtract c from both sides and divide by b.

I think the challenge, however, was in solving the linear equation by an unusual method. Trying to use the quadratic formula on the equation as it is presented gives a=0, which results in an indeterminate rational expression when using the formula. So - something clever needs to be incorporated in order to make the quadratic formula viable and still yield a correct solution to the problem.