A mass weighing 16 pounds stretches a spring damping force that is numerically equal to 8 3 feet. The mass is initially released from rest from a point 3 feet below the equilibrium position, and the subsequent motion takes place in a medium that offers a the instantaneous velocity. Find the equation of motion x(t) if the mass is driven by an external force equal to f(t) = 10 cos(3t). (Use g = 32 ft/s for the acceleration due to gravity.) x(t)= ft