Momentum is a vector quantity. In the absence of any losses, the momentum of the system after the collision is the same as that before the collision. So, taking north as positive the total momentum of the system is:
1300*30 - 900*15 = 25500 kg.m/s
.
After the collision the velocity of the two cars, assuming they are stuck together, is
v = 25500/(1300 + 900)
or v = 11.59 m/s (positive, so going north)
.
.