Решите уравнения: х+12-5х=88 3а+21а=100 2*х*15=120 18х-х+9х=104

х + 12 - 5х = 88                               3а + 21а = 100

- 4х = 88 - 12                                   24а = 100

- 4х = 76                                          а = 100 : 24

х = 76 : (- 4)                                     а = 4 4/24 

х = - 19                                            а = 4 1/6

2 * х * 15 = 120                               18х - х + 9х = 104

30х = 120                                        26х = 104

х = 120 : 30                                     х = 104 : 26  

х = 4                                                х = 4