The question says that #a in x = #b in y.
I think the answer is Σ*
let's take the members of Σ* and check
Σ* ={€,a,b,aa,ab,ba,bb,..........}
for €: 0 a's in x and 0 b's in y;
a: x=€ and y=a; 0 a's in x and 0 b's in y;
b: x=b and y=€; 0 a's in x and 0 b's in y;
aa: x=€and y=aa; 0 a's in x and 0 b's in y;
bb: x=bb and y=€; 0 a's in x and 0 b's in y;
ab: x=a and y = b ; 1 a in x and 1 b in y;
ba: x=b and y = a ; 0 a in x and 0 b in y;
and so on;
It is denoting the set of (a+b) *