Hi there!
Recall that:
Change in momentum = mass × change in velocity
Or:
Δp = mΔv = m(vf - vi)
Plug in the given values. We can assign east to be positive and west to be negative in this instance (Velocity is a vector with direction).
Thus:
Δp = (1)(-21 - 10) = -31 kgm/s OR 31 kgm/s WEST.
The correct answer is B.
Change in momentum is EQUIVALENT to the quantity of IMPULSE.
The correct answer is H.