It's coming wrong

By about how much will f(x,y,z) = In vx"+ y"+z" change if the point P(x,y,z) moves from Po(2,5,15) a distance of ds = 0.1 unit in the direction of 31 + 6] - 2k? . .. The change in value is about df ~ |0.0018 (Round to four decimal places.)