I'm not sure I understand what you are asking. If my answer is not applicable to your question, please post again with more detail.
If you wanted to solve for x:
r= y + x
subtract both sides by y
r - y= x
If you wanted to solve for y:
r= y + x
subtract both sides by x
r - x= y
r's value is currently (y + x)
Hope this helps anyway! :)
