I need some help , i doing my in formal methods (formal verification) i am facing a problem and need an expert opinion i have set say {s | f + g = k} where f and g are some functions

and k is number i want to split this set into two set (union or intersection) as {s | f = i} union (or intersection) {s | g = j} , first i want to ask can i do it , secondly what will be values of i & j (with respect to k), looking forward for some body who can help me out...