A mass weighing 16 pounds stretches a spring 8/3 feet. The mass is initially released from rest from a point 4 feet below the equilibrium position, and the subsequent motion takes place in a medium that offers a damping force that is numerically equal to 1/2 the instantaneous velocity.

Find the equation of motion x(t)

if the mass is driven by an external force equal to f(t) = 25 cos 3t.(Use g = 32 ft/s2 for the acceleration due to gravity.)