Two cars each of mass 1300 kg are approaching each other on a head on collision course. Each speedometer read 19m/s. What is the magnitude of the

Two cars each of mass 1300 kg are approaching each other on a head on collision course. Each speedometer read 19m/s. What is the magnitude of the total momentum of the system?I know the answer is 0 but I am confused as to what formula leads me to that answer. Is it -p1=p2?