This is calculus

In the article "Do Dogs Know Calculus?" the author Timothy Pennings explained how he noticed that when he threw a ball diagonally into Lake Michigan along a straight shoreline, his dog Elvis seemed to pick the optimal point in which to enter the water so as to minimize his time to reach the ball, as in the figure. He timed the dog and found Elvis could run at 6.6 m/s on the sand and swim at 0.87 m/s. If Tim stood at point A and threw the ball to a point B in the water, which was a perpendicular distance 10 m from point C on the shore, where C is a distance 15 m from where he stood, at what distance x from point C did Elvis enter the water if the dog effectively minimized his time to reach the ball? 10 D 15 (Use decimal notation. Give your answer to two decimal places.) Elvis entered water a distance of m from point C