Answer: <em>Learning a language can be difficult, yet it is one of the most useful hobbies that I've tried.</em>
Explanation:
A compound-complex sentence is a sentence which consists of two (or even more) independent clauses and at least one dependent clause. These clauses are joined by a conjunction. In this case, the independent clauses are: 1) "Learning a language can be difficult" and 2) "it is one of the most useful hobbies". These clauses are joined by a conjunction "yet", and a dependent clause "that I've tried" is added, which makes a compound-complex sentence.
