Answer
$v(a)=12a^{2}+6;$
$t=1\Rightarrow v\left( 1\right) =12\times 1^{2}+6=18\dfrac {m}{s} $
$v\left( 2\right) =12\times 2^{2}+6=54\dfrac {m}{s}$
$ v\left( 3\right) =12\times 3^{2}+6=114\dfrac {m}{s}$
Work Step by Step
$v=\dfrac {\partial s}{\partial t}=\dfrac {\partial l}{\partial t}\left( 4t^{3}+6t+2\right) =12t^{2}+6\Rightarrow t=a\Rightarrow v\left( a\right) =12a^{2}+6;$$t=1\Rightarrow v\left( 1\right) =12\times 1^{2}+6=18\dfrac {m}{s} $
$v\left( 2\right) =12\times 2^{2}+6=54\dfrac {m}{s}$
$
v\left( 3\right) =12\times 3^{2}+6=114\dfrac {m}{s}$