The values on the nutrition label from the question (11 g total carbs, 9 g sugars, 3 g fiber) could be actually explained by rounding:
- Total carbohydrates: 11.45 (rounded to 11)
- Sugars: 8.55 g (rounded to 9 g)
- Fiber: 2.55 g (rounded to 3 g)
...which would mean there are additional 0.35 grams of non-sugar, non-fiber carbohydrates...
...using such a label with only rounded values (and students not knowing the exact values), does not make it possible for students to calculate the amount of non-sugar, non-fiber carbohydrates.
So, another explanation is that there is an error in either total carbohydrates (11 g - too low) or in sugars (9 g - too high; more likely 6 g), according to NutritionData for 150 grams of raw onion:
- Total carbohydrates: 14 g
- Sugars: 6.4 g
- Fiber: 2.6 g
In either case, it seems they have used a nutrition label that is inappropriate to ask a question about the amount of non-sugar, non-fiber carbohydrates in raw onion.