We solve this inequality with a sign chart.
x^3<=4x^2+3x
x^3-4x^2-3x<=0
x(x^2-4x-3)<=0
We need the roots of the quadratic equation
x^2-4x-3=0
The discrimenant is
Delta=b^2-4ac=(-4)^2-4*(1)*(-3)=16+12=28
As, Delta>0, there are 2 real roots
x_2=(-b+sqrtDelta)/2=1/2(4+sqrt28)=2+sqrt7=4.646
x_1=(-b-sqrtDelta)/2=1/2(4+sqrt28)=2-sqrt7=-0.646
Let f(x)=x(x-x_1)(x-x_2)
We can build the sign chart
color(white)(aaaa)xcolor(white)(aaaa)-oocolor(white)(aaaa)x_1color(white)(aaaa)0color(white)(aaaaaa)x_2color(white)(aaaa)+oo
color(white)(aaaa)x_1color(white)(aaaaaaa)-color(white)(aaaa)+color(white)(aaaa)+color(white)(aaaa)+
color(white)(aaaa)xcolor(white)(aaaaaaaa)-color(white)(aaaa)-color(white)(aaaa)+color(white)(aaaa)+
color(white)(aaaa)x_2color(white)(aaaaaaa)-color(white)(aaaa)-color(white)(aaaa)-color(white)(aaaa)+
color(white)(aaaa)f(x)color(white)(aaaaaa)-color(white)(aaaa)+color(white)(aaaa)-color(white)(aaaa)+
Therefore,
f(x)<=0 when x in (-oo, 2-sqrt7] uu [0, 2+sqrt7]
graph{x^3-4x^2-3x [-12.34, 12.97, -9.01, 3.65]}