Maximum directional derivative and gradient

Question

Suppose that a function $f(x,y)$ has a gradient $[1,3]$ at a point $P$. Give a unit vector:

1. in the direction of the maximum rate of increase of $f$ at $P$
2. in the direction of the maximum rate of decrease of $f$ at $P$
3. in a direction for which the rate of exchange of $f$ at $P$ is zero

I have no idea how to approach this. I know how to solve this only if $f$ is given. Some explanations and/or exmples would be really helpful for this particular exercise, thank you.