Answer:
(3x + 7)²
Explanation:
You can either factor it by GCF or guess and check. The question you should be asking when you try to factor is"What multiplies to 49 and adds up to 42 with coefficients 3 in front of the x?" When you answer this question, you should think of 7 and 7. You then put it in binomial form and you have your answer.