We can say that the painted line is in a rectangular shape. Thus, the area is expressed as the product if its length and its width. We were given the area of 900 square feet and the width 1/3 foot. From the formula of the area, we are left only with one unknown therefore we can solve the problem.
Area = Length x width
900 ft^2 = Length x (1/3 foot)
Length = 2700 ft