Problem 4. A mass of 2.5 slug, when attached to a spring, stretches it 3.5 feet and then comes to rest in the equilibrium position. Starting at t = 0, an external force equal to f(t) = 4 sin 9t is applied to the system. Find the equation of motion if the surrounding medium offers a damping force that is numerically equal to 6 times the instantaneous velocity.