kinetic theory of gases momentum

[lukasz08]

p=momentum=(mass*volume)

delta px = pf - pi
mvf - mvi
mvx - (-mvx) <<<<---- where does this second negative come from ? what am i missing here?
2mvx


in my book it shows that
delta px = - mvx - (mvx) = -2mvx

[Doc Al]

When a particle bounces elastically off a wall, its velocity (and momentum) changes direction. Does that help?

[lukasz08]

yes i understand that so the second negative appears after the particle has hit the wall resulting in the 2mvx ?

[lukasz08]

ok i got it mvi = -mvf therefore -mvi - mvf = -2mvx