СРОЧНО!!!
х/12 - х/21 = 7/4
x/12-x/21=7/4
7x/84-4x/84=147/84
3x/84=147/84
3x=147
x=49
x/12 - x/21 = 7/4 домнножим на 84
7x - 4x = 147
3x = 147
x = 49