Find the directional derivative, D_vf, of f(x, y, z) = 2x tan^(-1)(√y z) at the point P = (1, 1, 1) in the direction of the vector v = i - 2j - 2k.
a. D_vf = -2/3
b. D_vf = 2/3
c. D_vf = 4/3
d. D_vf = -4/3