4/5 - 2/3а = 8/7 : 48/35,
4/5 - 2/3а = 8/7 * 35/48,
4/5 - 2/3а = 5/6,
2/3а = 4/5 - 5/6,
2/3а = 24/30 - 25/30,
2/3а = -1/30,
а = -1/30 : 2/3,
а = -1/30 * 3/2,
а = -1/20,
проверка:
4/5 - 2/3*(-1/20) = 8/7 : 48/35,
4/5 + 2/60 = 8/7 * 35/48,
48/60 + 2/60 = 8/7 * 35/48,
50/60 = 5/6,
5/6 = 5/6