Starting with 12 to 17, you added <u>5</u>. Then 17 and <u>7</u> to get 24. Then 24 and <u>9</u> to get 33, etc. You`ll notice that the bolded terms add by two. The systems continues just like that. You need to add 17 to 72, which gives you 89 as the 9th term. Hope this helps
