Calc 3 directional derivative question

Find the directional derivative of f (x, y, z) = z ln (x/y) at (1, 1, 2) toward the point (2, 2, 1).

What I did was find the distance between the two points to be the directional vector (i+j-k) and then I took the norm of the direction vector. so my unit vector = 1/sqrt(3) * u; then I found the gradient. From there, I found the scalar product of my unit vector and the gradient to get 0. Did I approach this problem right? Does that answer seem correct to you?