А)
(7/12 - х) : 3/8 = 2/7 : 9/14,
(7/12 - х) : 3/8 = 2/7 * 14/9,
(7/12 - х) : 3/8 = 4/9,
7/12 - х = 4/9 * 3/8,
7/12 - х = 1/6,
х = 7/12 - 1/6,
х = 7/12 - 2/12,
х = 5/12,
Б)
1/2∙х - 1/3 = 5/9,
1/2*х = 5/9 + 1/3,
1/2*х = 5/9 + 3/9,
1/2*х = 8/9,
х = 8/9 : 1/2,
х = 8/9 * 2/1,
х = 16/9,
х = 1 7/9