In[]:=
RegionPlot[ImplicitRegion[x==y^2,{x,y}]]​​
Out[]=
In[]:=
Show[Plot[x^2,{x,-5,10},PlotRange->{-10,10},PlotStyle->Red],ContourPlot[{x==y^2,x==3},{x,0,5},{y,-10,10}]]​​
Out[]=