Anony mous: but in reality, you have to look at the first part of the sentence - "I'm sure China can take care of the problem",
Yup, and yesterday China 'fixed' the problem by declaring:
"China will not allow chaos and war to break out on the Korean peninsula ..."
to a group of Asian foreign ministers.
There is no other answer to the problem.
Besides that (and maybe something that Trump had not even considered when he started talking nonsense), in a few days the ROK (South Korea) will elect a new President, to replace Park who was impeached for corruption, and removed from office.
The likely winner of the election is thought to be Moon Jae-in, who is expected to have a much more friendlier attitude to the DPRK than the impeached Park. Moon has criticised the recent deployment of the THAAD system by the USA, as this article in USA Today indicates:
Moon argued that the U.S. should have waited for a new president to make the final decision to install the $1 billion system, which the Pentagon apparently rushed to make operational for fear Moon might block it if elected.
"We express strong regret over the THAAD (installation) taking place in disregard of the people's will and procedures," Park Kwang-on, public relations chief of Moon's election campaign, said at a news briefing on April 26.Reference: https://www.usatoday.com/story/news/world/2017/05/05/us-relationship-risk-south-koreas-presidential-election/101330018/