Answer
$0.420\rm{ft}$
Work Step by Step
The length is $1.3$ miles which is the same as $1.3\times5280=6864$ feet.
The volume of the cylinder is $3800 \rm{ft}^3=\pi r^2h$. The height is 6864 and therefore
$3800=6864\pi r^2$. Solving for $r$ we get
$r=\sqrt{\frac{3800}{6864\pi}}\approx0.420\rm{ft}$