Suppose $f(x,y)=\sqrt[\phantom{2}]{tan(x)+y}$ and $u$ is the unit vector in the direction of $($:$0,-3$:$).$ Then,

$($a$)$ gradf

$($b$)$ gradf$(0\mathrm{.}2,6)=$

$($c$)f_u(0\mathrm{.}2,6)=D_{u}f(0\mathrm{.}2,6)=$