Question

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

Answer 1

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:
$( x^{2} -9x)+(xy-9y)$
Now work on finding a common thing between the 2 groups:
$x(x-9)+y(x-9)$
See, what you have in common there now is the (x-9) that you can factor out:
$(x-9)(x+y)$
And that is factored completely.