Simple but difficult for me. momentum, collsion

[teggenspiller]

Homework Statement

Two identical bodies are sliding toward each other on a frictionless surface. One moves at 1 m/s and the other at 2 m/s. They collide and stick. The magnitude of the velocity of the combined mass is
A. 3/4 m/s.
B. 2/3 m/s.
C. 1/6 m/s.
D. 1/2 m/s.
E. 1.5 m/s.
F. none of these

Homework Equations

p1=m1(v1f-v1i)+m2(v2f-v2i)=vf(m1+m2) i think that's right. {(?))

The Attempt at a Solution

ohkay, so using the idea behind the equation i just wrote.. i did

m's are irrelevant since theyre the same

so 1m/s + 2m/s = 3m/s
so at first thought i say okay its 3/3, so 1, that's not an option, so i picked F. Wrong.

then i did m(1) +m(2) = vf (m+m)

i picked 3/2, maybe the velocity over 2 masses? so 3/2=1.5m/s

Wrong. what the heck am i doing wrong, can anyone tell me how to approach a situation like this?

 

[teggenspiller]

hello everyone, please excuse this, as i accidentally posted it twice =X