A mass weighing16poundsstretches a spring8/3feet. The mass is initially released from rest from a point 2feetbelow the equilibrium position, and the subsequent motion takes place in a medium that offers a damping force that is numerically equal to the instantaneous velocity. Find the equation of motion if the mass is driven by an external force equal tof(t) = 10 cost(3t). [Hint: x'(t) = 0]