X^4 - x^2 + 9 - (x^3 + 3x^2 + 12) =
x^4 - x^3 - 4x^2 - 3
Polygon A,B,C,D,E and polygon L,M,N.O.K
determinant: 
(a) 
D<0 means there are no real roots. there are two complex roots with imaginary components.
(b) D=16+20=36>0
D>0 means there are two real roots
(c) D = 20^2-4*4*25 = 0
D=0 means there is one real root with multiplicity 2
It would be D hope this helps