Explanation:
3x + 1 is an expression. It is not an equation, so there is no solution.
There may be an answer expected, but it depends on the question.
Answer: 3x + 1 is an expression that is simplified as much as it can be. Therefore, either your question is incomplete or it's a trick question. If so, then the answer is 3x + 1.
4.3m questions
5.6m answers