Meaning of a negative value for non-fiber, non-sugar carbohydrate based on a nutrition label
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...

...BUT...

...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.