Chapter 12 - Section 12.2 - Arithmetic Sequences - 12.2 Exercises - Page 857: 50

$n=48$

$a_{n}=a+\left( n-1\right) d;a_{m}=a+\left( m-1\right) d\Rightarrow a_{n}-a_{m}=\left( n-m\right) d$ $a_{n}-a_{1}=\left( n-1\right) \times d\Rightarrow 2795-3500=\left( n-1\right) \times \left( -15\right) \Rightarrow n-1=\dfrac {-705}{-15}=47\Rightarrow n=48$