TO estimate this we can find the estimation answer 2 ways. The first way it to round each number to the nearest whole number and then subtract, and the 2nd way is to subtract and then round the answer. Lets do both:-
10.39 = 10
4.28 = 4
10 - 4 = 6
10.39 - 4.28 = 6.11 = 6
Hope I helped ya!!