Let f$($x$,$ y$,$ z$)=$ sin$($xy $+$ z$),$ P $=(0,1,$ $)$ and Q $=(1,1,1).$ a$)$ Calculate the directional derivative of f at P in the direction of the vector making an angle $2$ $3$ with $$f$($P$).$ b$)$ What is the unit normal vector of this surface at Q$?$