Answer
See image.
Work Step by Step
(Graphed with geogebra)
The level curves have the form:
$k=\sqrt{36-9x^{2}-4y^{2}}$
$9x^{2}+4y^{2}=36-k^{2}$
ellipses with y as the major axis.
(except when k=6, a circle)
The graph of f(x,y) is the upper half of an ellipsoid.