Chapter 12 - Section 12.2 - Arithmetic Sequences - 12.2 Exercises - Page 857: 49

$n=33$

$a_{n}=a+\left( n-1\right) d;a_{m}=a+\left( m-1\right) d\Rightarrow a_{n}-a_{m}=\left( n-m\right) d$ $a_{n}-a_{1}=\left( n-1\right) \times d\Rightarrow 601-25=\left( n-1\right) \times 18\Rightarrow n-1=\dfrac {576}{18}=32\Rightarrow n=33$