Question

Melissa is 9 years old. How old will she be in x years?

Answer 1

If you’re posing this question as x being a variable, then your answer would be any number times 9.

Ex:

9 + 3 = 12 yrs old

9 + 10 = 19 yrs old

9 + 2 = 11 yrs old

Answer 2

Taking that you didn't provide any other information about the values of x..

The only plausible answer, which probably isn't your case but still can be valid out of context is 19.

Explanation:

"X" is 10 in Roman numbers.

9 + 10 = 19

(But I suppose that you had more details to the question that you didn't yet share :) )

OR.. the answer might be an algebraic expression..

"9+x" years old.