ax^2+bx+c=0
x1+x2=-b/a
x1x2=c/a
-----------------------
6x^2+x-2=0
x1+x2=-1/6
x1x2=-2/6
1/x1^3+1/x2^3=(x1^3+x2^3)/(x1*x2)^3=(x1+x2)(x1^2-x1x2+x2^2)/(x1x2)^3=(x1+x2)[(x1+x2)^2-3x1x2]/(x1x2)^3
-1/6((-1/6)^2-3(-2/6)) : (-2/6)^3=-1/6(1/36+6/6): (-2/6)^3=-1/6*37/36 : -8/6^3=37/8