#2x^3 < x^2+3x#
#2x^3 - x^2-3x<0#
#x(2x^2 - x-3)<0#
#x=(-1+-sqrt(1-4*2*(-3)))/4#
#x=(-1+-sqrt(25))/4#
#x=(-1+-5)/4#
#x=-1 or x=-3/2#
The second-degree polynomial will be negative in# ]-3/2,-1[#
#color(white)(...........)-oo.....-3/2......-1........0............#
#xcolor(white)(...--)---------0++++++++#
#polynomial color(white)(.)+++0color(white)(.)--0+++++#
#gives color(white)(.)-----0color(white)(.)++0--0+++++#
#]-oo, -3/2[uu]0, +oo[#