If the figure you're describing is a rectangle, then you can use the relationship L = 99ft + W and the formula for the perimeter P = 2L + 2W. Substitute the equation for length in the perimeter formula to get P = 2×[99 ft +W] + 2W = [198 ft +2W]+2W = 198 ft + 4W. Now substitute in your value for the perimeter to get [858 ft] = 198 ft + 4W. Subtract 198 ft from both sides to get 660 ft = 4W. Divide both sides by 4 to get 165 ft = W. Now substitute that in your original formula to get L = 99 ft + [165 ft] = 264 ft.
