(3a+7a-8 -A- quadratic cube) -(2a-5a+2-3a- quadratic cube) -(2a-4 -A- quadratic cube)

=-a^3-3a^2+7a-8-3a^3+2a^2+5a-2+a^3+2a^2+4

=-3a^3+a^2+ 12a-6

=a^2(-3a+ 1)+6(2a- 1)

= 1/4*(-3/2+ 1)+0

=-( 1/4)*( 1/2)

=- 1/8

I wish you happiness.