Ответ:

1)3|5-x|=21

|5-x|=7

5-x=7

5-x=-7

x=-2

x=12

2)|x-4|×6=18

|x-4|=3

x-4=3

x-4=-3

x=7

x=1

3)2|4x-9|-5=1

2 x |4 x - 9 |= 1+ 5

2 x |4x - 9| = 6

|4x - 9 | = 3

4 x - 9 = 3

4x - 9 = - 3

x=3

x=3/2