hello! i have a problem about DEplot. can some of you help me to solve this problem? I use Maple 18. here the problem I've

restart;
with(DEtools);
 
>DE3:={diff(y(x),x)=y(x)-z(x),diff(z(x),x)=z(x)-2*y(x)};    
>DEplot(DE3,[y(x),z(x)],x=0..3,y=0..2,z=-4..4,arrows=large);

   when i enter it, I dont get the graphic. can you tell me why? thank you!

a1:=odeplot(p,[t,x(t)],0..5,color=black,thickness=2):

a2:=plot((2/125)*t^3-(3/25)*t^2+t+1,t=0..5,color=red,style=point,thickness=4):
display(a1,a2,labels=[x1,x2]);
Error, (in plots:-display) expecting plot structure but received: a1