6. Solve the initial value problem 3y" - 2y' - y =0, y(0) =0, y'(0) = -4. 7. Use the method of undetermined coefficients to solve the differential equation y" + 9y = re". 8. Use the method of undetermined coefficients to solve the initial value problem y" + 25y = cos(5x), y(0) = 3, y(1/2) = 0