f[x_] := x^3*Exp[x]

p1 := Plot[f[x], {x, -4, 2}, PlotRange -> All]

p2 := Plot[f'[x], {x, -4, 2}]

Show[p1, p2];

Solve[f'[x] == 0, x]

f[-4] // N

f[0]

f[2]

f[2.0]