First the displacement of the object is

(4, 9, 15) - (2, 3, 0) = (2, 6, 15)

which has a magnitude of sqrt(2^2 + 6^2 + 15^2) = 16.278820596

The magnitude of the direction vector of the force is

sqrt(5^2 + 9^2 + 3^2) = 10.723805295

The angle between the force and the displacement is

cos(t) = (5i + 9j - 3k) (dot) (2i + 6j + 15k)/(|5i + 9j - 3k|*|2i + 6j + 15k|)

cos(t) = (10 + 54 - 45)/(10.723805295*16.278820596) = 0.108838299

W = F (dot) s = |F|*|s|*cos(t) = (8 N)*(16.278820596 m)*0.108838299 = 14.174073147 J

Thus W = 14 J.

-Dan