In each of problems 11 through 28, find the general solution of the given differential equation. 11. y − y − y + y = 0 12. y − 3y + 3y − y = 0 13. 2y − 4y − 2y + 4y = 0 14. y(4) − 4y + 4y = 0 15. y(6) + y = 0 16. y(4) − 5y + 4y = 0 17. y(6) − 3y(4) + 3y − y = 0 18. y(6) − y = 0 19. y(5) − 3y(4) + 3y − 3y + 2y = 0 20. y(4) − 8y = 0 21. y(8) + 8y(4) + 16y = 0 22. y(4) + 2y