Think about addition in base two. Say a=10101 and b=1001. What your operation does is it modifies the bits in your numbers, so if the first bit in a is 1 and the first bit in b is 1 (as is the case above) you can make both 0 by making that bit 1 in x. This is actually the only way you can decrease the resulting sum, so x=1 is an answer above.

Noticing the hint above we now deduce x = a & b where & is bitwise and. So just printing (a^(a&b)) + (b^(a&b)) works, but there's an even nicer formula. We'll leave it up to you to prove that (a^(a&b)) + (b^(a&b)) = a ^ b :)