Is there a way to transform a function on the TI-89 in this manner?

1. The function is f(x,y)

2. I want to transform it to f(v) where v=y/x

For example, the function is f(x,y)=y/x and I want to write it as f(v)=v where v=y/x (although the function will be more complicated)

any help would be greatly appreciated! :)