Learn French: Ce problème n'est plus inquiétant. - This problem is no longer worrying.

inquiétant is a present participle (formed from the verb inquiéter, meaning 'to worry') being used as a verbal adjective. Because it acts just like a regular adjective in this sentence, it must agree in gender and number with the noun it describes. Since problème is masculine singular, inquiétant stays in its base form. If we were talking about a feminine noun like situation, it would become inquiétante.

What is the difference between inquiétant and inquiet?

They are related but have different meanings. Inquiet means 'worried' and describes how someone feels. Inquiétant means 'worrying' and describes the thing that is causing the worry. In this sentence, the problem is causing the worry, so it is inquiétant.

problème ends in an -e. Why do we use the masculine ce instead of cette?

Even though it ends in -e, problème is a masculine noun. Words of Greek origin ending in -ème are typically masculine in French. Because it is masculine and starts with a consonant, we use the masculine demonstrative adjective ce (meaning 'this').

How does the negative structure n'est plus work here?