Answer:
See below
Step-by-step explanation:
Using conservation of momentum:
Initial momentum = (77+32 kg)* 3.8 m/s = 414.2 kg m/s
Jumping off with zero speed relative to ground (I think this is what zero horizontal speed MEANS) means all of his momentum is given to the cart ( think about jumping off of a skateboard)
final momentum of the cart
mv = 414.2
v = 414.2/32 = 12.94 m/s for a + 9.13 m/s change