课外迁移阅读，回答相关问题
                                                                                                             看山中大雾
                                                                                                                   徐志摩
                                                                                这一瞬息的晨雾—— 那分明是山，水，田，庐；
                                                                                是山雾？ 又分明是悲，欢，喜，怒；
                                                                                是合幕？ 啊，这眼前刹那间的开朗
                                                                                这一转瞬的沉闷—— 我仿佛感悟了造化的无常！
                                                                                是云蒸？
                                                                                是人生？

如图，在△ABC中，DE∥BC，分别交AB，AC于点D、E．若AD=3，DB=2，BC=6，则DE的长为____ .