Homework:

Derive differentiation of continuity equation

$\frac{\text{d}\text{e}\text{l}}{dt}+\frac{\text{d}\text{e}\text{l}(u)}{\text{d}\text{e}\text{l}\text{x}}+\frac{\text{d}\text{e}\text{l}(u)}{\text{d}\text{e}\text{l}\text{y}}+\frac{\text{d}\text{e}\text{l}()}{\text{d}\text{e}\text{l}\text{z}}=0$