Answer
$5$
Work Step by Step
$a_{n}=a+\left( n-1\right) d\Rightarrow $
$a_{4}=a+\left( 4-1\right) d=a+3d;$
$a_{6}=a+\left( 6-1\right) d=a+5d$
$a_{6}-a_{1}=a+5d-\left( a+3d\right) =2d=17-11\Rightarrow d=3$
$a_{4}=a+3d=a+3\times 3=11\Rightarrow a=2$
$a_{2}=a+\left( 2-1\right) d=a+d=2+3=5$
