#### Answer

a) For all (x,y,z) except the plane (x,0,0) that is, $x$-axis;
b) For all $(x,y,z )$, except the plane (0,y,0) and (x,0,0), that is excluding x and y axes

#### Work Step by Step

a) There must not be zero in the denominator for that make sure $y\ne0$ and $ z \ne0$. Thus, all (x,y,z) except the plane (x,0,0).
b) There must not be zero in the denominator for that make sure $x\ne0$ and $ z \ne0$.Thus, for all $(x,y,z )$, except the plane (0,y,0) and (x,0,0) and excluding x- and y- axes