For Q3, take east at positive
Total initial momentum = 7*1200 - 3*1500 = 3900 (positive, so towards the east)
Therefore, as momentum is conserved, final momentum = initial momentum = 3900 Ns east
Final velocity of 1500kg cart = final velocity of stuck carts = v = p/m = 3900/(1500+1200) = 1.44...