Calculus?

You are travelling up a mountain and want to figure out the direction to head at to reach the top as soon as possible. Your altitude is given by the function $f(x,y)=-\dfrac{x^2}{5}-\dfrac{y^2}{5}+10$ and you are at the point $(x,y)=(5,5)$ . Give the magnitude of maximum altitude increase.

Graph

Note : Round to the nearest tenth