You need to review how to handle "reversed differences":
The shortcut is
No doubt that is why you got the sign wrong.
Anyway, let's do your problem:
Multiply the bottoms only:
Have you learned the shortcut trick about reversed differences?
That is, that (x-9) cancels into (9-x) and goes -1 times?
If you have, fine, then use it. But if you haven't learned that
trick, then write 9-x in descending order -x+9 then factor out -1
and get -1(x-9), like this:
Write 9-x in descending order as -x+9
Then factor -1 out of -x+9 which changes the signs:
Now you can cancel the (x-9)'s
Now you can substitute 9 for x:
Edwin
