6целых 10,21-3целых 5,14+4целых 5,6=

6 10/21 - 3 5/14 + 4 5/6  = (6 - 3 + 4) + (10/21- 5/14 + 5/6) = 
= 7 + (20/42 - 15/42 + 35/42) = 7 + 40/42 = 7целых 20/21. 

6 10/21-3 5/14+4 5/6=6 20/42-3 15/42+4 35/42=10 55/42-3 15/42=
=7 40/42=7 20/21