Show[Plot3D[{0,2 y,6},{x,-1,1},{y,0,2},Mesh->False,RegionFunction->Function[{x,y,z},(x^2)≤y≤2-(x^2)],BoxRatios->Automatic,PlotStyle->{Opacity[0.3],Opacity[0.3],Opacity[0.6]},BoundaryStyle->Thick,Filling->{2->{Top,LightBlue}}],Plot3D[{2 y,6},{x,-1,1},{y,0,2},Mesh->False,RegionFunction->Function[{x,y,z},(x^2)≤y≤2-(x^2)],BoxRatios->Automatic,PlotStyle->{LightBlue,LightBlue}]]

Plot3D[{2 y,6},{x,-1,1},{y,0,2},Mesh->False,RegionFunction->Function[{x,y,z},(x^2)≤y≤2-(x^2)],BoxRatios->Automatic,PlotStyle->{Opacity[0.3],Opacity[0.3]}]