Answer
$d=-3$
$a_{n}=14-3n$
$ a_{5}=14-3\times 5=-1$
$a_{100}=14-3\times 100=-286$
Work Step by Step
$d=a_{4+1}-a_{n}=8-11=5-8=2-5=-3$
$a_{n}=a+\left( n-1\right) d\Rightarrow a_{1}=a+\left( 1-1\right) d=a=11$
$a_{n}=a+\left( n-1\right) d=11+\left( n-1\right) \times \left( -3\right) =11-34+3=14-3n$
$a_{n}=14-3n\Rightarrow a_{5}=14-3\times 5=-1;a_{100}=14-3\times 100=-286$
