I can't answer the riddle.

Hint 1 of 4:

Remember, you're looking at a mirror.

Hint 2 of 4:

A mirror reverses everything.

Hint 3 of 4:

So XI + XI in a mirror is actually IX + IX. In roman numbers,
IX is 10 - 1 or 9. Therefore, the sum of 9 + 9 is the answer.

Hint 4 of 4:

18 in roman numerals is XVIII (10+5+1+1+1).