Trigonometry Help: A boat is heading towards a lighthouse, whose beacon-light is above the water

Full question:
A boat is heading towards a lighthouse, whose beacon-light is feet above the water. From point A, the boat’s crew measures the angle of elevation to the beacon, , before they draw closer. They measure the angle of elevation a second time from point B at some later time to be . Find the distance from point A to point B . Round your answer to the nearest foot if necessary.