Given $f(x,y)=e^{x}cosy$;$P(0,\frac{}{4})$;$,a=5i-2j$

a$)$ Find the directional derivative of $f(x,y)$ at $P$ in the direction of vector $a_{?}$

b$)$ Find a unit vector in the direction in which $f(x,y)$ has steepest $($maximum$)$ change at $P,$ and find the rate of change of $f(x,y)$ in that direction.