I can't seem to get this question...:

A ladder of length L standing on level ground is leaning against a vertical wall. The base of the ladder begins to slide away from the wall. What is the equation of the path followed by any point of the ladder? (Hint: let the distance from the top of the ladder to the point in question be K).

The answer is x^2/K^2 + y^2/(L-K)^2=1

...how?!

Thanks for your help