Why does the 2 disappear in the kinetic theory pressure derivation?

[MathewsMD]

http://en.wikipedia.org/wiki/Kinetic_theory#Pressure_and_kinetic_energy

Based on the derivation for P in the above link, one of the early steps was:

Δt = 2L/v_x

which is fine since it is analyzing one specific wall of the container, but then in the next step:

F = Δp/Δt = mv_x²/L

And the 2 seems to disappear completely. Could anyone please explain why it is gone?

 

[Simon Bridge]

The 2 cancels with the expression for the change in momentum (1st line).