6. Let f(x, y, z) = cos(3x + 42) + y'es and consider the point P = (7, 2, 0). (a) Find the directional derivative of f at P in the direction of 7 = (2, -1, 2). (b) In which direction u does f increase the fastest, starting at P? What is this maximum rate of increase? (c) In which direction does f decrease the fastest, starting at P