R[x_]:=-x^5+5x^3-4x
Plot[R[x],{x,-3,3},PlotRange->{-20,20}]

P[x_]:=(x+2)*x*(x-3)
Plot[P[x],{x,-5,5},PlotRange->{-20,20}]

P[x_]:=(x+2)*x^3*(x-3)^2
Plot[P[x],{x,-5,5},PlotRange->{-40,40}]

P[x_]:=3x^3-x^2+5x+1
Q[x_]:=3x^3
Plot[{P[x],Q[x]},{x,-2,2},PlotRange->{-30,30}]
Plot[{P[x],Q[x]},{x,-4,4},PlotRange->{-50,50}]