Extension: Directional Derivatives

Find the directional derivative $D_uf(x,y,z)$ if $f(x,y,z)=\frac{y}{z-x}$ and $\textbf{v}=<2,2,1>$ and $\bf{u}$ is the unit vector in the direction of $\bf{v}$. What is $D_uf(2,5,1)$?