Total momentum before collision=Total momentum after collision
(mass x velocity)+(mass x velocity)=(mass x velocity)+(mass x velocity)
(1100kg x 18m/s)+(1300kg x -16m/s)=total momentum
total momentum=-1000 kgm/s
Momentum means mass x velocity.
One car going north so I assume toward north is positive and toward south is negative.Actually it doesn't matter the other way if you want the north to be negative and south is positive but if it show a diagram or the question said which direction suppose to be negative or positive,you have to follow that.