-2 3/7+8 9/14+(-5 5/21)
-2 3/7 + 8 9/14 + (-5 5/21)= -2+8-5 + (9/14- 3/7 - 5/21) =
= 1 + (9*3-3*6-5*2)/42 = 1 + (27-18-10)/42 = 1- 1/42 = 41/42