A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... single fraction; the common denominator is 4a2. We get <math>\left(x+\frac{b}{2a}\right)^2=\frac{-4ac+b^2}{4a^2}=\frac{b^2-4ac}{4a^2}.</math> Taking ...