(3-2)  The rocks (each of the same mass m) have- before they collide - equal and opposite velocities in your frame of reference. So we can say:

Velocities:      v(A)  = -v(B) 

Momentum (p)  = mass x velocity, so:

           pA = mv(A)   and pB = mv(B)   =  -mv(A)

So total momentum = pA + pB  =  mv (A)  -  mv(A)   = 0  before the collision.

Because the identical rocks hit each other at equal speeds they exert identical forces upon one another and hence, after the collision, the velocity of each rock is simply reversed:

v+(A)  =  -v(A)     and     v+(B)  =  -v(B)  =  v(A) So now:

p+A = - mv(A)   and p+B = mv(A)

Pre-collision total momentum = p1 + p2  =   -mvu(A)  +  mvu(A)   = 0

Post-collision total momentum = p+A + p+B  =   -mv(A)  +  mv(A)   = 0    

The total momentum remains zero, so it is – as we knew it would be - conserved.