1) ( 6х/7 ) + 2 = ( 6х + 14 ) / 7
2) ( ( 6х + 14 ) / 7 ) / Х = ( 6х + 14 ) / 7х
3) ( ( 6х + 14 ) / 7х ) + 3 ( 1/21 ) = (( 6х + 14 ) / 7х ) + ( 64/3 ) = ( 3( 6х + 14 ) + 64•7х ) / ( 3•7х ) = ( 18х + 42 + 448х ) / 21х = ( 466х + 42 ) / 21х
4) ( 466х + 42 ) / 21х = 12 (8/21 ) • 0,8
5) 12 ( 8/21 ) • 0,8 = 260/21 • 4/5 = 208/21
6) ( 466х + 42 ) / 21х = 208/21
21( 466х + 42 ) = 21х•208
466х + 42 = 208х
258х = - 42
Х = - 42/258 = - 21/129 = - 7/43
Ответ ( - 7/43 )