Factor x2 - 18x + 81. show your work

To factorise it start with putting the xs in brackets.
(x )(x )
Then you must find 2 numbers which multiply to +81 and sum to -18.
81 is the product of 9×9 or -9×-9.
9+9=+18
-9+-9=-18
So they two numbers must be -9.
So factorised it is (x-9)(x-9) or (x-9)^2