Calculate Torque: Vector Products Homework

[shell4987]

Homework Statement

Force = (8.7 N) - (4.2 N) acts on a pebble with position vector = (3.7 m) - (6.5 m), relative to the origin. What is the resulting torque acting on the pebble about (a) the origin and (b) a point with coordinates (5.0 m, 0, -1.7 m)?

Homework Equations

The cross product torque=rxf
r'=r-d

The Attempt at a Solution

I got part (a) correct, however I cannot get part (b) correct, I've done it about 5 times and it is completely wrong, my answers are: 0i + 6.21j + 0k. Can anyone tell me what I'm doing wrong?

 

[Astronuc]

How did one do the first part?

I'm a bit confused as to the position vector (3.7 m) - (6.5 m) and the pebble location. Is the pebble at the origin and the position vector is the moment arm of the force?

 

[shell4987]

Force is Force = (8.7 N)i - (4.2 N)k and position vector = (3.7 m)j - (6.5 m)k, which is relative to the origin