Niagara Falls

A man went on a trip to see Niagara Falls. He got double-dared to swim horizontally from one side of the falls to the other, spanning $z$ meters. He knows that he can swim horizontally at a rate of $x ~ \frac{m}{s}$. The current, which moves directly downstream, carries him down the falls at a rate of $y~\frac{m}{s}$. How far away must the cliff of the falls be in order for him to make it to the other side? Provide your answer in the case where $x=5 ~\frac{m}{s}$, $y=20 ~\frac{m}{s}$, and $z=500~m$