Answer :
first simpliify
(3r^2+4)(2r-1)=
(3r^2+4)(2r)+(3r^2+4)(-1)=
(6r^3+8r)+(-3r^2-4)=
6r^3+8r-3r^2-4=
put back
6r^3-3r^2+8r-4-8r^3+6r^2=
-2r^3+3r^2+8r-4
(3r^2+4)(2r-1)=
(3r^2+4)(2r)+(3r^2+4)(-1)=
(6r^3+8r)+(-3r^2-4)=
6r^3+8r-3r^2-4=
put back
6r^3-3r^2+8r-4-8r^3+6r^2=
-2r^3+3r^2+8r-4