Answer
$4=x^2+y^2+z^2$
Work Step by Step
Remember that the level curve for $f(x,y,z)=\sqrt {x^2+y^2+z^2}$ has the form of $c=\sqrt {x^2+y^2+z^2}$ ...(1)
Here, we have $x=1,y=-1,z=\sqrt 2$
Then $c=\sqrt {(1)2+(-1)^2+(\sqrt 2)^2} \implies c=2 $
Thus, equation (1), becomes: $2=\sqrt {x^2+y^2+z^2}$
Hence, $4=x^2+y^2+z^2$
