Object A has mass mA $=10$ kg and initial momentum p$\_$a$,$i $=<18,-7,0>$ kg$*$m$/$s$,$ just before it strikes object B$,$ which has mass m$\_$B $=13$ kg$.$ Just before collision object B has initial momentum p$\_$B$,$i $=<6,7,0>$ kg$*$m$/$s$.($a$)$ Consider a system consisting of both objects A and B$.$ What is the total initial momentum of this system, just before the collision? $($b$)$ The forces that A and B exert on each other are very large but last for a very short time.