14 - 13,2 : (3 11/21 - 2 4/15) = 3,5
1) 3 11/21 - 2 4/15 = 3 55/105 - 2 28/105 = 1 27/105 = 1 9/35
2) 13,2 : 1 9/35 = 132/10 : 44/35 = 132/10 * 35/44 = 3/10 * 35 = 105/10 = 10,5
3) 14 - 10,5 = 3,5