First-order PDE $(\partial_x+\partial_y+\partial_z)f=x-y$ by characteristics

Find the most general solution of \(\left( { \frac { \partial } { \partial x } } + { \frac { \partial } { \partial y } } + { \frac { \partial } { \partial z } } \right) f ( x , y , z ) = x - y\)