Show[Plot3D[9-(y^2),{x,-3,3},{y,-3,3},Filling->Bottom,BoundaryStyle-> Blue,Mesh->False,PlotStyle->Opacity[0.2]],ParametricPlot3D[{0,y,λ(9-(y^2))},{y,-3,3},{λ,0,1},Mesh->False,PlotStyle-> Opacity[0.2],BoundaryStyle->{Thick,Blue}]]

Manipulate[Show[Plot3D[9-(y^2),{x,-3,3},{y,-3,3},Filling->Bottom,BoundaryStyle-> Blue,Mesh->False,PlotStyle->Opacity[0.2]],ParametricPlot3D[{t,y,λ(9-(y^2))},{y,-3,3},{λ,0,1},Mesh->False,PlotStyle-> Opacity[0.2],BoundaryStyle->{Thick,Blue}]],{t,-3,3}]