I down-voted, and flagged this answer with "other" with the following comment:
Not only is this a link-only answer which won't be very useful on its own if the answer on the other site disappears (or the site is down, or they go out of business, etc.), it points to a site that requires a paid subscription to see the answer. Boo.
For a long time we were using "not an answer" for these posts, but that is not quite accurate - they're answers, just not very good ones, because they're not self-contained. In this case they're even worse because they require paid subscription. This is like telling someone to go buy this month's Playboy magazine because the answer is on page 63. An answer, but again, not a very good one.
In a lot of cases we can comment before flagging, so the user can improve the content and make it a better, self-contained answer. In this specific case, however, as Servy points out, they may not be able to improve the answer because the content may not be legally copied to the answer here. But the answer is not a useful answer for our site because only those who have paid to see the content can get anything from it. This flies in the face of the whole purpose of our site.
Related / background:
Still having issues with link-only answer flags
Can we get some consensus on what flag to use for link only answers?
Is it forbidden to use "Other" flag for link-only answers?
Should I flag answers which contain only a link as "not an answer"?
Add a "link-only answer" flag reason
Should there be a policy about 'One-Link-Only-Answers'?