$($c$)$ If $u=log(x^3+y^3+z^3-3xyz),$ then prove that

$\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{x}}+\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{y}}+\frac{\text{d}\text{e}\text{l}\text{u}}{\text{d}\text{e}\text{l}\text{z}}=\frac{3}{x+y+z}$