2 1/7a+3 9/14a=11 1/7-1 1/21-2 3/14

2 1/7a + 3 9/14a = 11 1/7 - 1 1/21 - 2 3/14 2 2/14a + 3 9/14 = 11 6/42 - 1 2/42 - 2 9/42 5 11/14a = 9 39/42 или 9 13/14 a = 9 39/42 : 5 11/14 a = 1 58/81.