The Keller segel model used is [1] : $\frac{\partial A}{\partial t} = \bigtriangledown^2 A + k_A A(1 - \frac{A}{k_p}).$ When $a \ne 0$, there are two solutions to \(ax^2 + bx + c = 0\) and they are $$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$
Reference 1
