A vehicle towing a trailer through a spring-damper coupling hitch is shown in Fig. 2P-7. The following parameters and variables are defined: M is the mass of the trailer; Kh, the spring constant of the hitch; Bh, the viscous-damping coefficient of the hitch; B₁, the viscous-friction coefficient of the trailer; y₁(t), the displacement of the towing vehicle; y2(t),the displacement of the trailer; and f(t), the force of the towing vehicle.

Write the differential equation of the system.