Question 171185
The D they are referring to is the Determinant of the quad.  It is found by using the coefficents of the standard form. When D<0 you get 2 complex roots for a solution.
:
D = b2 - 4ac 

:
example of an equation that gives a negative D....{{{x^2-4x+13=0}}}

 D = b2 - 4ac 
= (-4)2 - 4(1)(13) = -36
:
solution{{{system(2+3i,2-3i)}}}

*[invoke quadratic "x", 1, -4, 13]