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B: Because r is the wall. Think of it this way.
The wall (R) 6.5(R) - 1 = 259 because if the (R) is the wall then it should stay next to the 6.5 because it's a variable describing the wall. Hope this helped!
6.5r - 1 = 259, because each row is 6.5 centimeters and you subtract 1 because the last row is 1 centimeter shorter:)