(3/5 - 1/7) - (13/15 - 1/2) - 1/21 =
= (3*7/5*7 - 1*5/7*5) - (13*2/15*2 - 1*15/2*15) - 1/21 =
= (21/35 - 5/35) - (26/30 - 15/30) - 1/21 = 16/35 - 11/30 - 1/21 =
= 16*30/35*30 - 11*35/30*35 - 1/21 = 480/1050 - 385/1050 - 1/21 =
= 95/1050 - 1/21 = 95:5/1050:5 - 1/21 = 19/210 - 1/21 =
= 19/210 - 1*10/21*10 = 19/21 - 10/210 = 9/210 = 9:3/210:3 = 3/70