> restart:

> x:=2*r*cos(t):

> y:=r*sin(t):

> z1:=4-y^2:

> a:=plot3d([x,y,z1],r=0..1,t=0..2*Pi,scaling=constrained,color=red):

> z2:=x^2+3*y^2:

> b:=plot3d([x,y,z2],r=0..1,t=0..2*Pi,scaling=constrained,color=green):

> with (plots):

> display(a,b);