I'm studying Calculus II at UFRN and I'm having a difficult time analyzing certain shapes in the 3D plane and so I decided to use Wolfram and its API to help me with my studies. This is a problem with an exercise list:
Iwouldliketoplotonthewolframsiteusingafunctionfor3DPlotingdocumentedandexemplifiedbyWolframitself,youcancheckoutthis link
a>. 
Myintuitionledmetopastethesuggestedinputintothedocumentationitselfandintotheexampleonthesitebutitdidnotwork,justtonoteitfollowstheinput:
Plot3D[{x^2+y^2,-x^2-y^2},{x,-2,2},{y,-2,2} RegionFunction->Function[{x,y,z},x^2+y^2