This question has the answers given already.
Perhaps what is wanted is the method.
Momentum is mass times velocity.
p = m v
Velocity is the first derivative of position with respect to time:
v = x' = dx/dt
v = -8 + 8t
v at 2s = -8 + 16 = 8 m/s
v at 4s = -8 + 32 = 24 m/s
Get the momentum by multiplying by m = 2 kg
giving p = 16 and 48 kg-m/s.
Acceleration is the change in velocity per unit time,
so the average acceleration is
a = (24 - 8) / 2 = 8 m/sec^2
Can also get the acceleration by differentiating the velocity with respect to time:
a = v' = x" = 8
and here a is constant.
Force is mass times acceleration,
f = m a = 2*8 = 16 N in SI units