Table[a,{a,1,13,3}   ]

Manipulate[
 {
Show[
Plot3D[18-(x^2+y^2), {x,-3,3},{y,-3,3},Mesh->False],
Plot3D[a, {x,-3,3},{y,-3,3},Mesh->False,MeshShading->False,PlotStyle->{Blue}]   ],
Show[
{ContourPlot[18-(x^2+y^2)==a, {x,-3,3},{y,-3,3},ContourStyle->{Thick,Blue,Dashing[{.03}]}, Contours->1],
ContourPlot[18-(x^2+y^2), {x,-3,3},{y,-3,3}, Contours->7,ContourShading->False ]
}]    }
,{a,1,13,3}   ]