Answer
Top part (above z=0) of an elliptic cone.
Work Step by Step
Trace in x=0: $z=|y|$.
Trace in y=0: $z=2|x|$.
Traces when z is below $0$: none (square roots are nonnegative).
Traces for z above 9: ellipses, $4x^{2}+y^{2}=k^{2}.$
Graph: top part of an elliptic cone.
