Y by A-Thought-Of-God, FaThEr-SqUiRrEl, samrocksnature, mathematicsy, centslordm, megarnie, jhu08, tiendung2006, ImSh95, Adventure10, Mango247, ItsBesi
Find all functions
defined on all real numbers and taking real values such that
for all real numbers 

![\[f(f(y)) + f(x - y) = f(xf(y) - x),\]](http://latex.artofproblemsolving.com/f/5/2/f528fc4047e21247663d4993913eadf346315be7.png)
