This is the equation for the solution of a second order polynomial of the form `aX^2+bX+C = 0` and the solution produces the root with the MINUS radical; i.e
from the formula `(-b +- sqrt(b^2 -4ac))/2a` this is the solution `(-b - sqrt(b^2 -4ac))/2a`
For a derivation of the quadratic formula, see the video: https://www.youtube.com/watch?v=Sidv3d1SEXs&feature=youtu.be