Find each equilibrium solution x(t)≡x0 of the given second-order differential equation x′′+f(x, x′)=0. Use a computer system or graphing calculator to construct a phase portrait and direction field for the equivalent first-order system x′=y, y′=−f(x, y). Thereby ascertain whether the critical point (x0,0) looks like a center, a saddle point, or a spiral point of this system.
x′′+2x′+x+4x3=0