A force of 2 pound stretches a spring 1 foot. With one end held fixed, a mass weighing 8 pounds is attached to the other end. The system is submerged in a fluid that imparts a damping force numerically equal to 3/2 times the instantaneous velocity. Initially, the mass is displaced 6 inches above the equilibrium position and released with a downward velocity of 3 ft/s. Find the equation of motion.