that's the distance between (-b,c) and (a,0)
distance bewteen points (x1,y1) and (x2,y2) is
\(D= \sqrt{(x1-x2)^2+(y1-y2)^2}\)
so
(-b,c) and (a,0)
\(D= \sqrt{(-b-a)^2+(c-0)^2}\)
\(D= \sqrt{b^2+2ab+a^2+c^2}\)
that is the lenght of the diagonal
we need more info if you want exact lenght
