Chapter 14: Partial Derivatives - Section 14.1 - Functions of Several Variables - Exercises 14.1 - Page 788: 61

$2=\sqrt {x-y}-\ln z$

Here, the level curve for $f(x,y,z)=\sqrt {x-y}-\ln z$ has the form of $c=\sqrt {x-y}-\ln z$ Also, $x=3,y=-1,z=1$ This implies that $c=\sqrt {3-(-1)}-\ln (1)$ or, $c=2 $ Hence, $c=\sqrt {x-y}-\ln z \implies 2=\sqrt {x-y}-\ln z$