The total momentum imparted to the wall in time $∆t$ on the area A of the wall due to molecules of velocity $V_x$ is  (n is the no of molecules 

The number of molecules with velocity v_x, hitting the wall in time $∆$t is
$=\frac{1}{2}\left({\mathrm{AV}}_{\mathrm{x}}\mathrm{\Delta t}\right)\mathrm{n}$
The momentum imparted to the wall due to
collision of each molecule = 2mv_x
$\therefore \mathrm{P}={\mathrm{nmv}}_{\mathrm{x}}^2\mathrm{A\Delta t}$