To find out how to do it, make up number for F and X.  Let's make
up F=14, and make up X=5.  If it were that way, the problem would be

You can solve that easily and get that she must be 9 years old.

Now we ask ourselves this question: "How did we get 9?"

The answer to that question is: "We subtracted 14-5 and got 9."

So instead of subtracting 14-5 for our made-up problem, we do a 
similar thing with our original problem and subtract F-X instead 
of 14-5.

Answer: F-X

[Note, when we subtract two numbers, like 14-5, we get a third 
number, like 9, but when we subtract two letters, like F-X, we 
just indicate that subtraction F-X and consider that, just as 
it is, "F-X", to be the third number.]

Edwin