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
the wavelength is to be 56.67 cm and I know that i should be applying the double slit equation for destructive interference.
-- The lenses of eyeglasses work because of refraction.
-- A pencil standing in a half-glass of water looks broken because of refraction.
-- The lenses and mirrors in telescopes and microscopes work because of refraction.
-- When the sun is setting and it looks squashed ... shorter and wider than a true circle ... that's caused by refraction of the sunlight through Earth's atmosphere.
They would just have to box in a higher weight class