Answer :

AL2006
If there's no friction and the wagon is free to move, then its momentum changes by 600 kg-meters per second.
olemakpadu
From Newton's Derivation:

Change in Momentum = Force * time
m(v - u) =  F* t
Momentum Change =  200 N * 3 seconds
                                     = 600 Ns    or 600 Kg.m/s.