To find the distance between the two docks, we can use the Law of Cosines, which states that for any triangle with sides a, b, and c, and an angle C opposite side c, the relationship is given by: c² = a² + b² - 2ab * cos(C). In this case, we have a = 400 feet, b = 300 feet, and C = 45°. Plugging in these values, we calculate: c² = 400² + 300² - 2(400)(300)cos(45°). First, we find cos(45°) which is √2/2. Thus, c² = 160000 + 90000 - 2(400)(300)(√2/2). This simplifies to c² = 160000 + 90000 - 120000√2. After calculating, we find that c is approximately 551 feet. Therefore, the correct answer is 551 feet.