How do you make this into a compound sentence? a. Lance Armstrong had retired from racing. b. He decided to return for one more Tour de France.
A compound sentence is made of two separate sentences, or two independent clauses. An independent clause has all of the components of a sentence. It contains a subject, a verb, and a complete thought. Each of your sentences, a and b, is an independent clause. This means that each one is a complete sentence on its own. Therefore, when you combine the two independent clauses it makes a compound sentence.
The one other element you need to combine these into a compound sentence is a coordinating conjunction. You can also use a semicolon (;). It serves the same purpose as a conjunction. The coordinating conjunctions you can use are for, and, nor, or, but, yet, and so.
Not every conjunction makes sense, however. You have to choose one that makes sense. In this case, the word but is a good fit.
Lance Armstrong had retired from racing, but he decided to return for one more Tour de France.
Notice that there is a comma before the conjunction. This is an important factor in combining the two clauses into a compound sentence.