Chapter 11 - Sequences; Induction; the Binomial Theorem - Chapter Review - Review Exercises - Page 859: 17

$35$

The $n^{th}$ term of an arithmetic sequence is given by the formula: $a_n = a_1 + (n-1) \ d(1)$ where, $a_1 = \ First \ Term$ and $ d = \ Common \ Difference$ We have: $a_1=3 $ and $ d=a_2-a_1=7-3=4$ Plug in $9$ for $n$ into equation (1) to obtain: $a_{9} =3+(9-1) 4 =35$