Answer
See image.
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Work Step by Step
Equate $f(x,y)=c$
for each value of c
$\sqrt{25-x^{2}-y^{2}}=c$
square both sides,
$25-x^{2}-y^{2}=c^{2}$
$x^{2}+y^{2}=(25-c^{2})$
We have concentric circles about the origin with radii $\sqrt{25-c^{2}}$
$\sqrt{25-0^{2}}=5,$
$\sqrt{25-1^{2}}=\sqrt{24}=2\sqrt{6}$
$\sqrt{25-2^{2}}=\sqrt{21},$
$\sqrt{25-3^{2}}=4,$
$\sqrt{25-4^{2}}=3,$
