well to help clarify the (+/-) confusion remember that l'Hopital's rule is only applied when the rational is indeterimant thus since the limit of the denominator is 0 then the limit of the numerator must also be 0, but this is only true when you use the (+) version of the quadratic equation because otherwise the numerator tends torward -2b as a goes to 0.  Thus using the + form of the quadradic equation gives the desired solution of c/b