3. Let f(x, y, z) = x - 23 = and let v (5,2,1). 2y a) Find the directional derivative of the function f at the point (1,2,3) in the direction of v. That is, compute Duf(1,2,3), where u is a unit vector in the same direction as v. b) Give the unit vector u which would cause Duf (1,2,3) to have as large a value as possible. c) Give the largest possible value of Duf (1,2,3) (which occurs when u is the vector from part b).