The best way to think about this question is to imagine the plan of the cube. Looking at this you would see that there are 6 identical squares. Therefore you should realise that to work this question out you should work out the area of one square (4m2) and multiply it by 6 (4 x 6 =24).
If it takes 9 minutes to paint 1 m2 of wood, painting 24m2 would take 9 x 24 minutes, which equals 216 minutes. As there are 60 minutes in an hour, this would take 3 hours and 36 minutes.