simplify

5/7 - 1/2 (1/3+3/7)

5/7 - 1/2(1/3+3/7) =

5/7 - 1/6 + 3/14 =
30/42 - 7/42 + 9/42 =
39/42 - 7/42 = 32/42 = 16/21.