# Level curves
implicit_plot(f(x,y)==0, (x,-2,2), (y,-2,2)) + implicit_plot(f(x,y)==3, (x,-2,2), (y,-2,2)) + implicit_plot(f(x,y)==6, (x,-2,2), (y,-2,2)) + implicit_plot(f(x,y)==9, (x,-2,2), (y,-2,2)) + implicit_plot(f(x,y)==12, (x,-2,2), (y,-2,2)) + implicit_plot(f(x,y)==15, (x,-2,2), (y,-2,2))