Answer:
17
Step-by-step explanation:
Look at any 2 adjacent numbers and find the difference between them.
Starting with numbers 1 and 2, subtract the first from the second:
That's a difference of 8, meaning the second number increased by 8.
Check the next numbers, 2 and 3:
It increased by 8 again. Now that we know for sure that each term in the sequence increases by 8 over the previous one, we can find the number in the green box. Just add 8 to the number before it: