阅读下面这首古诗，回答下面两题。
                                                                                                                                            闽中秋思
                                                                                                                                      （唐）杜荀鹤
                                                                                                                      雨匀紫菊丛丛色，风弄红蕉叶叶声。
                                                                                                                      北畔是山南畔海，只堪图画不堪行。

小明同学对生物学产生了浓厚的兴趣．以下是他在学习和生活过程中遇到的几个问题或现象，请你联系所学生物学知识帮他做出解释．

若实数x、y满足条件$\{\begin{array}{c}x+y\ge 0\\ x-y+3\ge 0\\ 0\le x\le 3\end{array}$则2x-y的最大值为（   ）