bjr
C = 5/7 - (2/7) /(-3)
[ (2/7) / (-3) = - 2/(7 x 3) ]
C = 5/7 + 2/(7 x 3) = (5 x 3)/(7 x 3) + 2/(7 x 3)
= 15/21 + 2/21
= 17/21