x^2-3x-6=0

x1=(3+кор.кв.(9+24))/2=3+кор.кв.(33))/2=(3+5.7446)/2=4.3723

x2=(3-кор.кв.(9+24))/2=3-кор.кв.(33))/2=(3-5.7446)/2=-1.3723

1/(x1^3)+1/(x2^3)=1/4.3723+1/(-1.3723)=-0.5