已知$\frac{a}{2}=\frac{b}{3}=\frac{c}{7}$，求$\frac{a+2b-3c}{2c-4b}$的值．

阅读下面古诗，完成后面题目。
                                                                                               江亭夜月送别二首（其二）
                                                                                                        王勃
                                                                                           乱烟笼碧砌^①，飞月向南端。

                                                                                           寂寞离亭掩， 江山此夜寒。
                                                                                           注：①碧砌：青石台阶。