Помогите пожалуйста с номером 540 б) и г)

Б) =(15/21 - 14у/21) 21=1            г)      7          2 4          1 3
     1у/21 21=1                                     -----m + --------m - -------m=7
         у=1                                                12          12          12
                                               
                                                              7m+8m-3m
                                                            -------------------=7
                                                                  12
                                                              
                                                                  12m
                                                                  ------=7
                                                                   12

                                                                    m=7