Assuming that ? is not an arithmetic operator (+, -, * / etc)
let us say ? is p, what is asked is:
xy*p = x * (x -y)
p = x * (x - y) / xy
=> p = x/y -1, in other words ? = x/y -1
putting this value back; x( x/y-1)y == x( x-y) ;)
Seems like a funny one to solve...