Momentum is conserved, so the sum of the separate momenta of the car and wagon is equal to the momentum of the combined system:
(1250 kg) ((36.2 i + 12.7 j ) m/s) + (448 kg) ((13.8 i + 10.2 j ) m/s) = ((1250 + 448) kg) v
where v is the velocity of the system. Solve for v :
v = ((1250 kg) ((36.2 i + 12.7 j ) m/s) + (448 kg) ((13.8 i + 10.2 j ) m/s)) / (1698 kg)
v ≈ (30.3 i + 12.0 j ) m/s