To add or subtract fractions, we need to have a common denominator.

8/15 + 2/15
Since the fractions already have a common denominator of 15, we can just add the numerators.
(8 + 2) / 15 = 10 / 15 = 2/3

-9/17 - 21/34
The common denominator for 17 and 34 is 34.
(-9 * 2)/34 - 21/34 = -18/34 - 21/34 = -39/34

-5/54 - 7/9
The common denominator for 54 and 9 is 54.
(-5/54) - (7 * 6)/54 = -5/54 - 42/54 = -47/54

-1/3 + 8/21
The common denominator for 3 and 21 is 21.
(-1 * 7)/21 + 8/21 = -7/21 + 8/21 = 1/21

Therefore, the results are: