Answer: The question asks to round to the nearest whole number and THEN find the difference. Your answer should be 3.
6.3 = 6
2.8 = 3
6 - 3 = 3
If the question was asking to round the difference to the nearest whole number your answer should be 4.
6.3 - 2.8 = 3.5
3.5 rounded to the nearest whole number: 4
Hope this helps!