If the third term is 31, then let's get the second term. We have to use the rule we were given and work backwards. So, we will add three and then divide by 2.
31 + 3 = 34
34 / 2 = 17
17 is the second term. Let's do the same thing we just did to find the first term: add three, divide by 2.
17 + 3 = 20
20 / 2 = 10
Answer: the first term is 10
Hope this helps!