Answer with Explanation:
We are given that
Force acts on a pebble=
N
Position vector=
m
a.We have to find the resulting torque on the pebble about origin.
Torque=
Substitute the values then we get

Torque=
N-m
By using 
b.

Torque about point (2,0,-3)

N-m
Drink lots of water, take breaks in the shade
Answer:
momentum=mass×velocity
momentum =400kg×20m/s=8000kg.m/s