If $x,y$ and $z$ are directions in a Cartesian coordinate system and $i$, $j$ and $k$ are the respective unit vectors, the directional derivative of the function $u\left ( x,y,z \right )=x^{2}-3yz$ at the point $\left ( 2,0,-4 \right )$ in the direction $\left ( i+j-2k \right )/\sqrt{6}$_____________ (rounded off to two decimal places).