2nd Order Nonlinear Differential Equation

Hi all,
I have some problems with an equation that tried to describe the displacement in x direction of a piston moved by a constant pressure. On the opposite side, to counter the movement, I have:

• the mass load
• a spring
• the inertia
• a counter-pressure that is function of the velocity at square.

I'm not able to resolve the equation.
I've removed the units of measure in the eventuality they could interfere in the process.

I use MC 15.

Regards