Answer:
(a) 2400N
(b) different
Step-by-step explanation:
2.5 milliseconds = 0.0025 seconds
a) The momentum of the moving arm-hand of 1.5 kg and at 4m/s speed is the product of the mass and the speed

After the slap, the arm-hand comes to rest, making the change in momentum of 6 - 0 = 6 kgm/s. This change in momentum is due to the impulse generated by the force within t = 0.0025 seconds.
So the average force is:

b) If there are 2 hands clapping together at same speed and mass, their total momentum must be double of that. This makes the average force 2 times as before.