Answer
$2=\sqrt {x-y}-\ln z$
Work Step by Step
Remember that the level curve for $f(x,y,z)=\sqrt {x-y}-\ln z$ has the form of $c=\sqrt {x-y}-\ln z$ ...(1)
Here, we have $x=3,y=-1,z=1$
Then $c=\sqrt {3-(-1)}-\ln (1) \implies c=2 $
Thus, equation (1), becomes: $2=\sqrt {x-y}-\ln z$
