A mass of 1 slug is attached to a spring whose constant is 15lb/ft. initially, the mass is released 1 foot below the equillibrium position with a donward velocity of 5ft/s, and the subsequent motion takes place in a medium that offers a damping force that is numerically equal to 2 times the instantaneous velocity. Question: find the equation of motion if the mass is driven by an external force equal to f(t)=12 cos2t +3sin2t