The area to be covered with wallpaper is (12m)(3m) = 36m2.
Each roll of wallpaper has area (6m)(0.75m) = 4.5 m2
Number of rolls = 36/4.5 = 8
The answer given assumes that the pieces of wallpaper match perfectly with no waste. Usually not realistic. Due to matching the pattern, extra wallpaper may be needed.