$\text{find the gradient field}$ $\vec{F}=\vec{V}\varphi$

$\text{for the following functions $\varphi$}\\$

$\text{a. $\displaystyle\vec{V}(x,y)=\tan^{-1}\left(\frac{y}{x} \right)$}\\$

$\text{b. $\displaystyle\vec{V}(x,y,z)=\cos\left(\frac{y}{x} \right)$}$

ok this completely new to me