Draw a sketch to represent the situation. As you can see, the rock is in free fall from y (the height of the clift to the ground) to the ground (y = 0). The soundt travels with uniform speed the distance y (upward). Let t1 be the time of the free fall and t2 the time of the sound traveling up. Then, the total time is t = t1 + t2 = ?. Then, use the free fall formula to get time t1 and then solve for the height of the clift.