Answer: If my mental math is correct, the answer is 3a + 4.
Step-by-step explanation: You would substitute the x in 3x +1 with 3(a+1) +1. Then you would do distributive property by multiplying 3 times (a + 1) which would give you 3a + 3. After that you would add 3 + 1 and get 4. So, the answer is 3a + 4. Hopefully my answer is correct and helped you.!!!!