9 5/21 - 10 2/3 - 3 5/28

To subtract the given fractions, we first need to convert them to improper fractions.

9 5/21 can be written as (9 * 21 + 5)/21 = (189 + 5)/21 = 194/21. Similarly, 10 2/3 can be written as (10 * 3 + 2)/3 = (30 + 2)/3 = 32/3. And 3 5/28 can be written as (3 * 28 + 5)/28 = (84 + 5)/28 = 89/28.

Now we can subtract the fractions:

194/21 - 32/3 - 89/28.

To subtract fractions, we need to have a common denominator. In this case, the least common multiple of 21, 3, and 28 is 84. So we need to rewrite the fractions as:

(194/21) * (4/4) = 776/84, (32/3) * (28/28) = 896/84, and (89/28) * (3/3) = 267/84.

Now we can subtract the fractions:

776/84 - 896/84 - 267/84.

To subtract fractions with the same denominator, subtract the numerators:

(776 - 896 - 267)/84.

Simplifying the numerator:

(776 - 896 - 267) = -387.

The final result is -387/84.