Need : write the expression in a complete factored form: x²-9x+xy-9y

Factor this by grouping.  Group the first two terms together and then the second 2 terms together.  The idea here is that, after grouping, what you can pull out in both groups is the same.  Like this:

Now work on finding a common thing between the 2 groups:

See, what you have in common there now is the (x-9) that you can factor out:

And that is factored completely.