There is no code to do that. x and y sum to (x+y) and that may or may not equal z. For example, if x=3 and y=9, there's no way that their sum will equal 42 instead of 12. 12 is what the sum is . If you need to clarify, like you want a new x and y that have the same relative ratio and those sum to z (as shown below), then please do clarify.
xNew = z * xOld / (xOld+yOld);
yNew = z * yOld / (xOld+yOld);
