a, 3/5 + 7/21 - (-4/5) + 7/5
= 3/5 + 1/3 + 4/5 + 7/5
= 14/5 + 1/3
= 42/15 + 5/15
= 47/15
b, -3/17 - (2/3 - 3/17)
= -3/17 - 2/3 + 3/17
= 0 - 2/3
= -2/3
c, -5/21 - (16/21 + 1)
= -5/21 - 16/21 - 1
= -1 - 1
= -2
d, 5/7 + 9/23 + (-12/7) + 14/23
= (5/7 - 12/7) + (9/23 + 14/23)
= -1 + 1
= 0