Problem 4. (Directional Derivative) Suppose f(x, y) = 10-22 - y2 + xy is the height of a mountain above a point (x, y) on the ground plane. A helicopter is hovering over the mountain above the point (x, y) = (1, 1), and it's about to hit the mountain if it continues to move in its current velocity without changing its height. In order to avoid the collision as quickly as possible, in which direction u the helicopter should move if it keeps its current height? (Hint: u as a direction vector has to be a unit one, i.e. its magnitude is 1.)