Answer
$d=-15$
$a_{n}=79-15n$
$ a_{5}=4$
$a_{100}=-1421$
Work Step by Step
$d=a_{n+1}-a_{n}=19-34=34-49=49-64=-15$
$a_{n}=a+\left( n-1\right) d\Rightarrow a_{1}=a+\left( 1-1\right) d=a=64$
$a_{n}=a+\left( n-1\right) d=64+\left( n-1\right) \times \left( -15\right) =64-15n+15=79-15n$
$a_{n}=79-15n\Rightarrow a_{5}=79-15\times 5=4;a_{100}=79-15\times 100=-1421$
