Why do we have to do (2 x 10!) instead of just (10!) ?
The question asks for passwords that are AT LEAST 9 digits long, so we need to look at two cases: a password with 9 digits and a password with 10 digits.
We do not need to look at any other cases (like a 11 digits) because we would end up repeating digits in the password, which we are told is not allowed.