Physics homework-momentum and impulse-

hello I'm struggling with physics this year at school i found this site looks great if this is a bad 1st post i wont be to surprised any way heres my problem...
1. The problem statement, all variables and given/known data

A Car A (with a mass of 750Kg) is travailing at 30 m s-1 and collides head on with car B travailing at 20 m s-1 with a (mass 1600kg) in the opposite direction


2. Relevant equations
A Calculate the magnitude of the momentum of each car before crashing

B If the two cars lock together in the crash, in which direction will they be moving immediately after the collision?

C
I Calculate the initial speed of the combined wreck immediately after the collision

II why dose the speed quickly reduce to zero?

3. The attempt at a solution

.......ok so far i have drawn a picture like so...

(carA)----------------><---------(carB)
|750kg|-----|30ms-1|-------|20ms-1|--|1600kg|

lol i have no idea...also i have the answerers in the back of the book but thats not learning so thats why i came here :)

any help would be great! even if you could point me in the right direction

Thanks
housedog1