Which is a run-on sentence?

 - Mr. Vance wrote the problem on the board, Mitch found the answer.
 - The three explorers in the big forest.
Mr. Vance wrote the problem on the board, Mitch found the answer is a run-on sentence. It has two sentences that are joined by just a comma: Mr. Vance wrote the problem on the board and Mitch found the answer.