x=linspace(-3.5,3.5,100);
y=pr03b_fce(x);
plot(x,y,x,0);

hold on
r1=fzero('pr03b_fce',-3);
r2=fzero('pr03b_fce',-1);
r3=fzero('pr03b_fce',3);
plot(r1,pr03b_fce(r1),'x',r2,pr03b_fce(r2),'x',r3,pr03b_fce(r3),'x');
hold off