Basically, your first step is to divide the total area of the walls by the area of the strips of wallpaper.
460/22 = 20.90909
But because we are using significant digits, we'll need to round up to where theatre are only 2 significant digits in the answer. So the final answer is 21.
I would advise that you look up significant digits on the internet because there are many rules that go along with it.