Answer
$l=64.5mi$
Work Step by Step
$k=860\frac{mi}{h^2}$
$h=35,000ft\times\frac{1mi}{5280ft}=6.629mi$
$v=300\frac{mi}{h}$
Minimum $l$ is at maximum vertical acceleration.
$\frac{6hv^2}{l^2}=k$
$l^2=\frac{6hv^2}{k}$
$l=v\sqrt{\frac{6h}{k}}=(300\frac{mi}{h})\sqrt{\frac{6(6.629mi)}{860\frac{mi}{h^2}}}=64.5mi$
