Question 1034193
I would like to help you on this one (you posted it twice) but since wallpaper goes on walls that have heights, and you did not supply the heights of the walls, I cannot help you.
If all you want is
R = 2L + 2W = 2(5.6) + 2(4.8) = 11.2 + 9.6 = 20.8 m

Your use of 2.5 isn't clear...did you want to divide the total above by 2.5?
If so, 20.8/2.5 = 8.32 rolls...he would need to buy 9 rolls...